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THE EVOLUTION OF SEXUAL STRATEGY
IN MODERN HUMANS: AN
INTERDISCIPLINARY
APPROACH
____________
A Thesis
Presented
to the Faculty of
California State University, Chico
____________
In Partial Fulfillment
of the Requirements for the Degree
Master of Arts
in
Interdisciplinary Studies
____________
by
Kendra Marie Collins
Spring 2014
THE EVOLUTION OF SEXUAL STRATEGY
IN MODERN HUMANS: AN
INTERDISCIPLINARY
APPROACH
A Thesis
by
Kendra Marie Collins
Spring 2014
APPROVED BY THE DEAN OF GRADUATE STUDIES
AND VICE PROVOST OF RESEARCH:
_______________________________
Eun K. Park, Ph.D.
APPROVED BY THE GRADUATE ADVISORY COMMITTEE:
_______________________________
Sharon Barrios, Ph.D.
Graduate Coordinator
_______________________________
Eddie Vela, Ph.D., Chair
_______________________________
Lawrence Herringer, Ph.D.
_______________________________
Kristina Schierenbeck, Ph.D.
PUBLICATION RIGHTS
No portion of this thesis may be reprinted or reproduced in any manner
unacceptable to the usual copyright restrictions without the written permission of the
author.
iii
TABLE OF CONTENTS
PAGE
Publication Rights .....................................................................................................
iii
Dedication .................................................................................................................
vi
Acknowledgments .....................................................................................................
vii
List of Tables.............................................................................................................
vii
Abstract .....................................................................................................................
ix
CHAPTER
I. Introduction .................................................................................................
1
Background ..........................................................................................
Economics of Evolved Sexual Strategy Differentiation ......................
Prenatal Hormones and Sexual Differentiation of the Body and
Brain ..................................................................................................
1
4
II. Literature Review ........................................................................................
11
Darwin’s Theory of Evolution .............................................................
Rationale andBiology
Hypotheses
Evolutionary
Theories Support the Evolutionary Pressure of
Sexual Selection ................................................................................
Evolutionary Anthropological Theories Shaping our Sexual Strategy
Evolution ...........................................................................................
Conclusion: Early Pressures Shaped Differential Sexual Strategies....
Evolutionary Psychology’s Short- vs. Long-term Strategies ...............
2D:4D Ratio, Hormones, and Sexual Strategy ....................................
Research Aims .....................................................................................
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N
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CHAPTER
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III. Methodology ...............................................................................................
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Participants...........................................................................................
Measures ..............................................................................................
Procedures ............................................................................................
Statistical Analysis ...............................................................................
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IV. Findings and Results ...................................................................................
45
Testing Research Question 1 - Do Males and Females Differ on their
Sexual Behaviors? .............................................................................
Testing Research Question 2 – Does 2D:4D Serve as a Biological
Marker for Differential Prenatal Hormones and Sexual Behavior? ..
Testing Research Question 3 – Are there “Norms” Between Males
and Females Shaped by Culture that Account for their Sexual
Strategy? ............................................................................................
45
V. Conclusions and Recommendations ............................................................
56
Limitations ...........................................................................................
References .................................................................................................................
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Appendices ................................................................................................................
A.
B.
Sexual Behavior and Attitude Survey..................................................
Demographic Questionnaire ................................................................
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DEDICATION
I’d like to dedicate this thesis to my best friend, the man who believed in me
enough to marry me. Thank you Charlie for the motivation to move forward, the patience
for my slow pace, and the million and one ways you pushed me to finish! Looking
forward to saying good-bye to that black cloud once and for all.
vi
ACKNOWLEDGEMENTS
There are so many very important people who need thanking – more than I
can name! Of course thanks to my wonderful husband Charlie who continued to
motivate me in as many ways as he could imagine. My dad who gave me confidence and
my mom who gave me strength. To Jessica who kept me accountable and Kelly who
gave me inspiration. To my committee (Dr. Vela, Dr. Herringer and Dr. Schierenbeck)
for their patience with the many starts and stops of this project and sticking with me to
the end. To Gwen and Sharon for the help navigating all the tricky paperwork. And
finally to all my friends and peers who had kind words of encouragement to keep me
focused, moving forward, and helping me celebrate my little wins along the way.
vii
LIST OF TABLES
TABLE
PAGE
1. Independent samples t-test of males’ vs. females’ relationship preferences
and actual and ideal sexual behaviors ……………………………………
47
2. Paired samples t-test comparing males’ and females’ actual and ideal
sexual behaviors………………………………………………………….
49
3. Correlations of males and females left hand 2D:4D with actual and idea
sexual behaviors………………………………………………………….
51
4. Independent samples t-test of males’ vs. females’ 2D:4D ratio and
relationship preference…………………………………………………… 53
5. Independent samples t-test of males’ vs. females’ BSRI standard and
individual item score…………………………………………………… ..
54
6. Left and right hand 2D:4D correlated with BSRI standard and individual
item scores for males, females, and males and females combined………. 55
viii
ABSTRACT
THE EVOLUTION OF SEXUAL STRATEGY
IN MODERN HUMANS: AN
INTERDISCIPLINARY
APPROACH
by
Kendra Marie Collins
Master of Arts in Interdisciplinary Studies
California State University, Chico
Spring 2014
Much research and theorizing using an evolutionary framework has been done
to study and understand how and why males and females vary in terms of sexual
behavior, specifically when it comes to differences in sexual strategy. The body of work
on evolved sexual strategy is often confined to and shared within a single field with
limited discourse across fields. The current study’s hypotheses were formulated to test if
theories of evolved sexual strategy from different fields held true, while using an
evolutionary psychology research design and method. Differences in behaviors,
cognitions, and physical features were found between males and females that support the
current body of research of evolved sexual strategies. Apart from giving further support
for evolved sexual strategy, the current study can be used as a model to study other
research questions in an interdisciplinary way.
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CHAPTER I
INTRODUCTION
Background
Research on human sexuality traverses a variety of fields, including biology,
anthropology, psychology, philosophy, history and sociology. Although methods vary
across different fields, the focus of this research often aims to investigate how males and
females differ in their sexuality, and can include details on developmental differences
(Bjorkquvist, Lagerspetz, & Kaukiainen, 1992), cultural differences (Costa et al., 2001),
cognitive or behavioral differences (Putrevu, 2001) or motivation (Forret & Dougherty,
2004). In the fields of biology, anthropology and psychology, this body of research is
frequently referenced as sexual strategy research.
History of Sexual Strategy Theory
The body of work accumulated in sexual strategy research in humans,
specifically in regard to differences between males and females, is nothing novel when
compared to the body of work amassed on other creatures. Darwin’s book, The Descent
of Man and Selection in Relation to Sex (1871), focuses exclusively on sexual selection,
dedicating only 70 pages to human sexual selection and 500 pages on sexual selection in
other animals. In this work, Darwin concludes that, across species, males are eager to
pair with as many females as possible, while females are much more selective when
choosing a mate. In countless studies of species from various taxa, this idea has held up;
1
2
one gender (usually males) appears more driven for quantity and variance of sexual
encounters while the other gender (usually females) is more selective and choosy about
their sexual encounters (Trivers, 1972).
One of the earliest experimental studies to demonstrate these effects was
conducted by Bateman (1948) who mixed up several flies that were heterozygotic for a
single, dominant trait. Upon tallying up the characteristics of the resulting offspring, he
assigned which of the parental generation successfully mated. The tallies indicated that
males and females did not copulate in equal frequencies; most females mated one time
while most males did not mate at all and only a few males mated several times. Barring
recent criticism of Bateman’s experimental techniques and statistical analysis (Snyder &
Gowaty, 2007), his findings pushed forward the idea that males are indiscriminate when
it comes to sexual partners and frequency, while females are more “choosy.” This same
effect has been found countless times and covers a wide range in the world of animal
taxa.
Further, studies have found that there is also intra-sexual competition between
males for increased opportunities to mate with a higher number of females (Buss, 1988)
and even competition between females to mate and/or couple with high valued male
partners (Rosvall, 2011; Clutton-Brock et al., 2006). These patterns have been found in
small mammals (Bond & Wolff, 1999), large mammals (Mysterud et al., 2005), birds
(Collias & Collias, 1970; Jacobs et al., 1978; Collias & Victoria, 1978), primates and
monkeys (Plavcan & van Schaik, 1992), and even reptiles (Arak, 1983). The above
results found that males competed for the favors of females, which led to opportunities to
reproduce, while the females selected the most desirable mate based on the “winner” of
3
this competition. These findings tie into Darwin’s observations that male sexual
objectives revolve around gaining the largest advantage in terms of number of partners,
and female sexual objectives revolve around selecting fewer (or one) of the best mates.
Sexual Strategy and Evolution
To understand the root causes of gender differentiation (which includes
differences between human males and females), many researchers nest sexual strategy
research within an evolutionary framework (Buss & Schmitt, 1993). When it comes to
sexual strategies, evolutionary theory posits that the differentiation present today is a
result of a long history of selective pressures that shaped and molded the basic principles
of sexuality, resulting in the sexual phenotypes (observable characteristics or traits) we
see today. Much like the evolved differentiation of beaks in Darwin’s finches, different
sexual strategies gave males and females specific advantages in their ability to
successfully reproduce. If those strategies had a genetic component, those who found
sexual (thus reproductive) success would have the opportunity to pass on those successful
strategies to their offspring, who in turn would also have the reproductive advantage
again to pass on those same sets of genes. Unsuccessful strategies, in likewise fashion,
would meet a dead end as the parental generation would not find reproductive success
and would not pass on those strategies to future generations.
The environment in which our ancestors evolved the traits and tendencies we
see today is called the Environment of Evolutionary Adaptedness (EEA; Foley, 1995)
and contains the long-standing and recurrent reproductive problems our ancestors faced
as a collective environment. The EEA cannot be thought of as a specific place, time or
setting; rather, it is the collective composite of a species’ environment which includes its
4
unique selective pressures faced over time. Much like the biological niche, no two
species’ EEA’s are alike, even if those species overlap in space and time (Tooby &
Cosmides, 1990).
Today, the modern human’s genetic make-up is the sum product of its own
EEA where those successful strategies selected by the pressures of our ancestor’s
environment persist. However, for humans, the EEA’s selective pressures were in a past
time when our environment was quite different than the modern environment we
currently occupy. For example, snakes and spiders were a common variable in human
EEA and it was necessary for humans to avoid these possible poisonous creatures to
enhance survival. Today, things like cars and electrical outlets cause more fatal dangers
than snakes and spiders, and yet, it is snakes and spiders people have innate, and
sometimes debilitating, aversions to which begin, almost uniformly, at age two (Öhman
& Mineka, 2001). This poses an interesting question to study in the field of human
sexuality: How do evolved sexual strategies, shaped by genes selected in an ancestral
environment, manifest in the modern age? According to the EEA and the evolutionary
framework, we should see patterns in sexual strategy that helped our ancestors gain the
best reproductive edge, but like phobias, may no longer serve the same reproductive
advantages.
Economics of Evolved Sexual
Strategy Differentiation
The reasons for differentiation of sexual strategies among humans and why
they may be crucial can be described using some simple economic comparisons of
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biological facts about males and females: 1) the size and potential number of the gender’s
gametes; 2) the minimal time investment in offspring; 3) the potential consequences of
gestation and; 4) the differential need for resources. The production of a single human
child requires input from both a male and a female, but the minimum requirement from
each gender of their gamete and parental investment are starkly different. In humans,
males can produce an astronomical amount of mobile gametes (sperm), at a rate of about
45 million per day (Johnson et al., 1980) while females only have about 400-500 gametes
(eggs) from birth (Zuckerman, 19511).
Once a single egg and single sperm fuse through sexual reproduction, a ninemonth incubation time is necessary within the womb of the female. Ultimately, a female
must invest, at the absolute minimum, nine months (i.e., pregnancy) plus the time needed
for copulation to reproduce. On the other hand, a male needs to invest, at minimum, the
time for copulation. In addition, a successful fusing of gametes leading to pregnancy also
differentially impairs the male and female parent from other mating opportunities leading
to reproduction; the male is able to successfully reproduce with other females while the
female will not be able to reproduce until, at minimum, the pregnancy ends (although
frequently longer due to the effects of lactation). Further, males do not need additional
resources during a pregnancy while a woman must “eat for two” and must secure
additional nutritional resources, even though she may be encumbered by the very
pregnancy that is necessitating these additional resources.
1
New research has demonstrated it is possible that mice may produce additional eggs
after birth (Johnson et al., 2004), but additional research is needed to better understand if
these post-birth eggs are viable and what their role in reproduction (if any) is.
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Finally, the risks of childbirth are not devoid of bodily and physical harm;
childbirth can be, most certainly, deadly for the female (roughly 2.5 – 8.0% of mothers
died in “unassisted childbirth” during a period from 1800-1950, before modern medicine
greatly reduced the rate of maternal mortality [Loudon, 1986]). Further, the birthing
process itself puts a woman in a very vulnerable position to other dangers, including
possible predators (historically speaking). No such threat exists to a man during
pregnancy or childbirth
The conclusions one can draw from such a comparison is that females, with
limited opportunities (eggs) to reproduce, need to invest exponentially larger amounts of
time. Males, on the other hand, with seemingly unlimited opportunities to reproduce
(sperm), need to invest a relatively small amount of time (Trivers, 1972). Males, who are
reproductively rewarded for seeking out several mating opportunities (both high
frequencies of sexual acts as well as sexual partners), will have offspring that likewise
seek out reproductive advantages with the result that the male’s “reproductive strategy”
will be to seek out as many sexual opportunities with as many sexual partners as possible.
Females, whose strategy would be to reproduce with the right partner at the right time
(because the wrong partner at the wrong time would, at minimum, put her at a
disadvantage, and at most, could result in her death), would have offspring that would be
relatively more selective in their choosing of sexual partners. These economic strategies
can account for both intra- and inter- sexual selection because male attempts for access to
sexual partners and encounters is balanced by female attempts to select among the most
high-valued mates, theoretically prompting males to compete amongst themselves to
demonstrate their high value to potential female mates (Bateson, 1983).
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Prenatal Hormones and Sexual Differentiation
of the Body and Brain
Hormones, most certainly, play a role in shaping what is considered “male”
(i.e., evolved tendencies to seek out lots of sexual partners) and “female” (i.e., evolved
tendencies to seek out few, high-valued sexual partners) sexual strategies. The endocrine
(hormonal) system is primarily responsible for sexual differentiation (Stjernholm, 2012)
during both pubertal and prenatal development. By the 12th week of pregnancy, male
external genitalia becomes distinguishable from female genitalia due to the production of
androgens within the testes (Nef & Parada, 2000). Testosterone, one of the most potent
androgens, is signaled for production by genes on the Y chromosome, the chromosome
boys inherit from their father and the single distinctive chromosome between males and
females. Without the genes from the Y chromosome to signal the production of
testosterone and its metabolites, the undifferentiated zygote would continue to
development into a female, which is the “default” gender (Nef & Parada, 2000).
In one of the earliest studies to document the sexual differentiation in the
womb, Phoenix and colleagues (1959) injected pregnant guinea pigs with a regimented
amount of testosterone throughout their pregnancy. The litter born, when compared with
litters from untreated mothers, contained all males, at least according to their external
genitalia. Interestingly, laparotomies (dissection through the abdomen) performed on the
litters of treated mothers occurring one week after birth showed that some of the
externally “male” guinea pigs in fact had intact internal female genitalia (ovaries, uterus,
etc.) and were biological females. So while these guinea pigs had the genetic make-up of
a female, the circulating male hormones actually redirected the natural progression from
8
female to male, at least in terms of their external genitalia. In essence, the fluctuating
male hormones replaced the non-existing Y chromosome and the resulting genetic
females became hermaphroditic.
Not only do hormones dictate the formation of physical features (as described
above with both normal and abnormal development), they also inform differentiation of
the brain and quite possibly, later behavior. True to this fact, the differences between
males’ and females’ brains are quite extensive. From the overall volume and size (males’
are larger overall with a larger amygdala but a smaller hippocampus [Goldstein et al.,
2001]), differentiation of regions when completing tasks (such as a memory task [Cahill,
2006]) to the structural differences (e.g., in the areas associated with visual cortex,
possibly giving rise to males’ capacity to process information and detection of motion in
a way to map out space and time; [Amunts et al., 2007]), males and females vary a great
deal in the ways of shape and function. That these differences in the brain are played out
in behavioral differences have been tested.
In addition to the physical differences Phoenix and colleagues (1959) found in
the hermaphroditic individuals, these individuals also expressed male behaviors during
adolescence and adulthood, like mounting. Studies with rhesus monkeys also treated
with testosterone during pregnancy had female offspring that expressed more maletypical behaviors as juveniles (e.g., more rough and tumble play) and as adults with more
male-typical and less female-typical behaviors (Goy & McEwen, 1980). There have been
countless studies confirming the results of testosterone’s effect on females either directly
by manipulating testosterone levels during pregnancy or by proxy. For example, female
rats adjacent to male rats in utero showed distinct male characteristics as juveniles and
9
adults such as mating and impregnating later, mounting other females and exhibiting
increased aggression towards others (e.g., Ryan & Vandenbergh, 2002).
In studies of humans, direct manipulations of hormones during the prenatal
period are unethical for the purposes of research, so studies relying on proximal
differences are necessary. Studies using twins have demonstrated that the female of
opposite-sex twin pair (one male and one female twin) show more masculinized patterns
of behavior when compared with either female of a dizygotic, same-sex female twin pair
(Culbert et al., 2008), indicating the testosterone the female twin is exposed to in utero
from her male co-twin has an effect on her later traits and behaviors. Studies of
individuals with disorders of sex development, such as girls with adrenal hyperplasia
(CAH) resulting in high levels of androgens in utero, again show similar patterns of
increased male-typical behaviors (Hines et al., 2004; Hines, 2009; Huges et al., 2006).
An additional proximal measurement of in utero testosterone exposure that
has been commonly used is 2D:4D finger measurements. 2D:4D is the ratio of a person’s
second (index) and fourth (ring) finger and is posited to be the result of prenatal
androgens (Lutchmaya et al., 2004) where a lower 2D:4D ratio (i.e., a longer fourth digit)
indicates higher levels of prenatal androgens and thus a more “masculine” 2D:4D. It has
been linked to a variety of sexually dimorphic characteristics and behaviors including
physical fitness, (Honekopp et al., 2006), behaviors in simulated “ultimatum” games
(Van den Bergh & Dewitte, 2006) and eating disorders (Klump et al., 2006). Advanced
measuring techniques have standardized and increased power to detect differences,
especially when 2D:4D is measured in this more precise way by trained experts
10
(Honekopp & Watson, 2010), especially when aided by the use of digitized techniques
(Voracek et al., 2007).
The current study aims to add to the growing body of research of sexually
evolved strategies and how they play out in the cognitions, behaviors and held beliefs in
modern society. If males and females have evolved differential sexual strategies to
increase their own sexual reproductive fitness, we can make predictions about how males
and females will differ in various sexual behaviors and cognitions. Importantly, the
current study will attempt to tie these variables into a tangible biological basis; a
participant’s 2D:4D ratio. By including a fixed biological variable, research can start to
untangle and understand the vehicles used by evolution to move the Homo sapiens
species forward in evolutionary space and time (i.e. EEA).
CHAPTER II
LITERATURE REVIEW
Introduction
“Nothing in Biology Makes Sense Except in the Light of Evolution”
Theodosius Dobzhansky (American Biology Teacher, 1973)
In his now famous essay, Theodosius Dobzhansky argued passionately that
Darwinian evolution is the only way to explain all of the wonderful diversity we see in
the world (Dobzhansy, 1973). Other scientific theories that aim to explain the diversity
in the world today (e.g., the length of a giraffe’s neck) have been held to the rigors of the
scientific community and it is Darwin’s theory of natural selection that has been left
standing.
One topic of interest, for Darwin and numerous researchers since, is the
evolved differences between males and females. There are countless curious differences
between the sexes, and researchers from many different fields, including biology,
anthropology, and psychology, have used theories of evolution to help account for and
explain these curiosities. Having researchers from different domains study the same
topic comes with benefits, especially when researchers know to seek out those benefits by
using a cross-disciplinary research framework.
A variety of philosophical paradigms allow researchers across disciplines to
study phenomena such as evolved sex differences with a diverse set of tools and expertise
11
12
This allows for the possibility to find solutions to problems that other fields, with
different sets of tools, may not be able to identify. It is also possible that different fields
studying the same topic will independently come to the same or similar conclusions,
which only strengthens the argument at hand. However, research not utilizing crossdisciplinary inquiries cannot cull these benefits and research can potentially suffer in two
ways. First, the amount of scientific literature one can draw from is restricted, thus
limiting the discoveries, methods, and theories of other fields’ work which can lead to the
“reinvention of the wheel” across different fields. Secondly, the limited flow of outgoing
research to other fields insulates a researcher, and her field, which could ultimately make
the work less viable in academic circles (Kloos, 2005).
There are three “levels” of cross-disciplinary research; multi-, inter-, and
trans-disciplinary. Multidisciplinary research involves researchers in different fields
studying the same problem, independently, using their own perspective. Interdisciplinary
research involves researchers from different fields working collaboratively across fields
to solve the same problem, but maintaining their perspective. Finally, transdisciplinary
research is the result of different research perspectives melding together to create an
entirely new theory or methodology and can be either a collaborative effort between
many researchers, or a solo effort by one researcher melding many perspectives (Neal,
2013). Cross-disciplinary research, regardless of the level, benefits the knowledge base
for the disciplines involved; further, it can create something new by crossing the
boundaries between different research philosophies, identifying these differences and
similarities along with strengths and weaknesses, and using ideas from each field to
develop new solutions or ideas.
13
The aim of this literature review is to take an interdisciplinary approach to
create a rationale to test the potential consequences of evolved differences between males
and females, in an effort to understand their different, present-day sexual strategies. In
addition to differences between genders, time will be spent looking at potential
differences in sexual strategy within males and females and look for any biological
factors that could predict these differences. First, I will lay out a brief history of
evolutionary theory from the fields of biology and anthropology. The topics briefly
covered will include Darwinian natural and sexual selection, some key theories from the
biological sciences supporting the ability of sexual selection to exert evolutionary change,
and some theories from the anthropological sciences representing key evolutionary
changes that could account for our current day sexual strategies. Second, I will introduce
a newer field, evolutionary psychology (EP), and its effort to use why based questions in
understanding if evolved sexual strategies can be accounted for by short- and long-term
mating strategies. Finally, I will present a potential biological marker, 2D:4D, that can be
used to predict how individual sexual strategies may differ.
Darwin’s Theory of Evolution
The voyage of the HMS Beagle
Any conversation about evolution should begin with a review of Darwin’s
originally proposed theory of natural selection. At the age of 22, and after a few failed
career attempts, Darwin’s interest in biological and geological sciences landed him the
unpaid position of naturalist and gentleman companion for Captain Robert Fitzroy’s
voyage on the H.M.S. Beagle. The Beagle’s five-year journey around South America,
14
Australia, Africa, and many of the islands between, began in late 1831 and proved to be
the jumping-off point for Darwin’s ground breaking theory of evolution (although his
book, On the Origin of Species, would not be published for nearly three decades, in
1859). During the five-year trek, Darwin read from the expansive library of 400 books,
witnessed intriguing landscapes suggesting long spans of time (e.g., a band of rock high
on a volcanic rock that included sea shells, evidence that the volcano had begun in the sea
and was slowly rising up), collected over 5,000 animal and plant species (many of which
had never been seen), and took copious amounts of notes in journals and diaries.
Arguably, the most well-known finding of Darwin were, what Darwin originally
described as, the “mix” of blackbird, gros-beaks, and finches from the Galápagos Islands.
These “mixed” birds were given to the renowned ornithologist John Gould
upon return of the Beagle who, on closer inspection, determined that the birds were, in
fact, 12 new species of finches. Of interest, and the reason Darwin originally thought
they were different types of birds, was the varied shape and size of beaks which seemed
highly specialized for available food on the island inhabited by each species. This
evidence, along with geological findings, other animal species and the extensive readings
Darwin had completed, gave him grave concern over the commonly held belief that
species were fixed and unchanging. Darwin found himself questioning the theory of the
“stability of the species.”
Natural selection
By 1837, a year after his return from the Beagle voyage, Darwin was well
underway in noting his ideas about evolution in personal journals, which he was calling,
15
“descent with modification.” The basic premise of natural selection included three key
components; variation, inheritance, and selection. Variation refers to the amount of
differences of any specific characteristic within individuals of a species (e.g., speed, beak
length, bone structure, etc.). Inheritance speaks to the idea that it is possible some of
these variable traits could be reliably passed down to offspring. It is only these traits that
can be reliably passed down (not, for example, an injury that changes a physical
characteristic) that play a role in the evolutionary process. Finally, selection is the idea
that some individuals with favorably inherited variations are “selected” for, as they are
more successful at survival and reproduction. Selection is the successful combination of
inheriting the “good” variations in a population (Buss, 2004).
His postulations that would lead to the three components was augmented after
he read Thomas Malthus’ 1798 publication, Essay on the Principles of Population. In
this book, Malthus, a reverend and economist, argued that humans had the capacity for
exponential population growth. Malthus pointed out that this exponential rate could only
occur under optimal environmental conditions, or, an environment with unlimited
resources and space to accommodate an ever-expanding population. This idea was key
and crucial to Darwin’s explanation on the mechanism of change over time. Darwin
interpreted Malthus’ writings to mean the environment sets an intangible boundary on a
possible population limit which in turn limits the potential population. A species’
population has the capacity for great numbers that will only continue to grow each
generation. However, because we don’t see those population numbers, there must be
some environmental pressures that weed out unfit members of the species who fail to
reproduce.
16
Herein lies the core of Darwin’s argument and the mechanism for change;
those pressures are not completely random. Some individuals of a species had some
characteristics that assisted their survival to a reproductive age. Any advantage that the
successfully reproducing members had was somehow passed on to future generations.
Meaning, the traits that were successful in assisting with reproducing in one generation,
were then passed on to offspring which in turn would help them reproduce and pass down
the trait once again to another generation who would have the trait and on and on.
Sexual Selection
Darwin’s theory of natural selection accounted for much of the phenotypic
(i.e. visible trait expression) variance in the natural world, however, there were oddities
left that were not explained by the three core principles of natural selection. For
example, male peacocks have elaborate tail plumage and bright blue-green bodies, with
the tail being prominently displayed during courtship rituals with the rather plain grey or
brown female peahens. This bright and burdensome display does not confer any obvious
survival benefits; indeed, it is seemingly counterproductive to natural selection as it tends
to attract predators, hinders the peacock’s escape, and requires a large amount of
nutritional resources to maintain. Darwin surmised that there must be additional selective
forces at work beyond natural selection and called the set of principles sexual selection
(Darwin, 1871).
Sexual selection, in comparison to natural selection, is the ability for some
members of a species to out-compete for sexual mates as compared with out-competing
for survival. In other words, sexual selection is the result from the struggle to reproduce
while natural selection is the result from the struggle to survive. Sexual selection theory
17
includes inter- and intrasexual selection. Intrasexual selection is the competition between
members of one sex (usually the males) to drive away, kill, or otherwise dispose of
sexual rivals. Intersexual selection is the competition between members of one sex
(again, usually males) to “win over” (i.e. attract) partners of the opposite sex (Buss,
2004).
Simply put, males compete against one another to actively put other males at a
sexual disadvantage (intrasexual selection) and/or demonstrate their value as sexual
partners, while females choose between those males based on the winners and/or those
with the highest perceived sexual value (intersexual selection)2. We see this expressed
across species, for example, in longer horn lengths in bighorn sheep used to battle other
males for access to females (Coltman et al., 2002) or nest size in barn swallows, where
males build elaborate nests and females select their sexual partner based on the size of the
nest, despite the fact that the size of the nest does not offer any survival benefit (Soler et
al., 1998).
Despite never knowing the actual mechanisms for genetic inheritance, Darwin
thoughtfully laid out a strong foundational starting point with a plethora of examples that
adhered to his theories of natural and sexual selection. The fields of evolutionary
biology, anthropology and psychology have since used these theories as a starting point
and have continued to build upon this fundamental framework. In the subsequent
sections, I will briefly explore the ways in which biological, anthropological, and
psychological evolution have utilized Darwin’s ideas to pinpoint how males and females
2
Although there are examples of reversed sex-role species where females compete for the opportunity to
mate with males, I will refer to males as the dominant/competitive species that compete for the females
throughout this paper for ease of readability.
18
evolved under different sets of evolutionary pressures and describe the potential outcome
of those evolutionary pressures.
Evolutionary Biology Theories Support the Evolutionary
Pressure of Sexual Selection
Ronald Fisher, one of the founders of population genetics, was interested in
Darwin’s theory of sexual selection and worked to explain how inter-sexual selection
could create sexually dimorphic traits (differences in phenotypic traits between males and
females) that gave no obvious survival benefit. His “runaway” theory is best explained
as a positive feedback loop with two components in which males, who have a desirable
sexually dimorphic trait, are selected more often by females who desire that trait, and
thus have more opportunities to mate. The two components were the “liking” and the
“having” of a sexually dimorphic trait where the females liked a trait that a male had
(Fisher, 1915; 1930). That is, the more the trait is selected for in one generation (more
liked), the more extreme the expression in future generations become (more haved),
which in turn, can perpetuate the gene throughout subsequent generations with females
choosing males that not only express the trait, but potentially express even more extreme
versions of the trait.
There are many possible examples to demonstrate how this theory could play
out, and two are illustrated here. Peacocks (male peafowl) have elaborate body coloring
and an amazing display of fanned tail feathers used during courtship rituals. If his dance,
and tail feathers are an elaborate enough display, the peahen will allow the peacock to
mate with her. Researchers have found that both the largest fanned tail feathers, as well
19
as tail feathers with more eye-spots, produce more offspring (Petrie et al., 1991).
Therefore peahens prefer, or “like”, the most elaborate display and peacocks with more
elaborate displays have greater reproductive success and these two components (the
liking and the having) are potentially passed down to future generations. The Irish elk’s
extinction was theorized to be due, at least in part, to runaway sexual selection. Female
Irish elk selected male elk with larger bodies as well as increasingly larger antlers. Some
recovered antlers measure nine feet across and weigh 90 pounds! The extreme nutritious
cost to grow such massive antlers, coupled with the burden of such a heavy load, may
have been more than the males could handle, particularly as their food source density
decreased during environmental changes. In this case, natural selection would have
favored males with smaller bodies and antlers needing fewer nutritional resources,
however the sexual selection pressures were so strong, and had become so fixed in a
positive feedback loop, that it ultimately cost the loss of the entire species (Moen et al.,
1999).
Fisher’s theory, while providing a general framework of one component of
sexual selection, lacked some important explanations. Namely, why would the females in
a species consider costly displays as “attractive” and select males that possessed the trait
as reproductive partners? Since they garnered no apparent benefit to either the male
expressing the trait or the female selecting for that trait, how could the trait perpetuate?
Amotz Zahavi, an Israeli evolutionary biologist, proposed an answer and described what
he called the “handicap principle” (Zahavi, 1975). The handicap principle states that the
elaborate displays some males possess are simply an honest cue females can use to gauge
the “fit” of a potential male sexual mate. The term honest is used because Zahavi
20
proposed that these showy displays, whether the color of plumage, size of tusks or
antlers, call of a bird, or other odd behaviors, hinted at some superior genetic quality not
otherwise available or apparent to those selecting a mate.
The fact that an individual can gather the resources necessary to survive the
perils of a large and showy display must mean that they have more than enough “good”
genes to survive, despite the handicap of their showy secondary trait. By selecting the
most developed secondary characteristics, a female would be choosing the most optimal
genotype (an individual’s genetic makeup); one that cannot only survive at the basic level
of acquiring food, water, and shelter, but has enough to do so while also carrying a
burdensome trait.
In sum, the theories presented build a strong case that sexual selection is, in
fact, a strong selection force that can favor seemingly odd secondary sexual
characteristics while maintaining genetic variability in a population. These favored
secondary sexual characteristics do not need to give any obvious benefits to survival, nor
even obvious benefits to reproduction but at their core, should give the individual an
advantage because they indicate a superior set of genes that a mate would desire for their
own offspring. These characteristics can include both physical manifestations, like
plumage or coloring, or can take the form of behaviors, like ornamental nest building or
physical feats of strength. Importantly, for these secondary sexual cues to drive sexual
selection, there must be an ability to choose a mate based on those cues. The ability to
successfully use external cues to assess internal genetic quality is something that could
increase our human ancestor’s ability to improve their sexual reproduction and will be
discussed more thoroughly in later sections.
21
Evolutionary Anthropological Theories Shaping
our Sexual Strategy Evolution
The field of evolutionary anthropology focuses on hominid and non-hominid
primates and their biological and cultural evolution (including similarities and differences
in their morphology, ecology, and behavior), both past and present. It includes an
emphasis of how humans evolved within a context of social living and how the pressures
of living within a group of related and non-related individuals shape human behavior
(Godkin, 2011). The theories presented here are of interest to study as they serve as clues
into how our evolutionary past shaped human sexual strategies of today.
Bipedalism (walking on two legs) and encephalization (increasing brain size)
are the two adaptations that evolved around the same time make humans unique
compared to other non-human hominids. The two traits, although incredibly beneficial,
created a life-threatening situation to females called the Obstetric Dilemma (Washburn,
1960). The Obstetric Dilemma is the evolutionary dilemma our ancestral lineage faced as
these two traits continued to evolve; as the hominin pelvis began shrinking to
accommodate walking, the skull began increasing to accommodate the growing brain
(McHenry, 1982). The inverse changes in growth posed a serious problem for females
who had to birth babies with increasing heads as their own pelvis size was decreasing.
The beginning of the Obstetric Dilemma, a shift that moved our ancestral
lineage distinctly away from our ape and chimp relatives, likely began with early
hominins, like Australopithecus, who had bipedal locomotion but only a slightly
increased cranial size. This shift would ultimately result in two very different birthing
patterns; those of modern chimps/apes and those of modern humans. Modern chimps and
22
apes are not considered to have an Obstetric Dilemma as they rarely have difficulties
giving birth. The ease of the birthing process is a result of several key features: 1) the
fetal head was, at most, the size of the maternal pelvis; 2) the plane of the birth canal runs
parallel such that the orientation of the birth canal from entry to end is consistent, thus the
fetus does not need to rotate as it ascends the canal; 3) the fetus’ orientation as it emerges
from the birth canal allows its head to face towards the mother who can clear out the
baby’s mouth and nose as well as unwrap or unwind the umbilical cord if it impedes in
the birthing process; and 4) the fetus is so well developed it can actually assist in its own
birth once its hands are clear of the mother’s pelvis. Because the birthing process is so
infrequently fatal, chimp and ape mothers can and do seek privacy for the birthing
process and the birth happens in complete seclusion. This is all in contrast to the difficult
human birthing process, complicated by the fetal head being slightly larger than the
maternal pelvis, the fetus’ need to complete several rotations through the birth canal’s
differing orientations, the human fetus facing away from the mother when born,
necessitating at least one “helper” in the birthing process who can clear the fetus’ airways
and untangle the umbilical cord, and finally that once born, the fetus is completely
helpless and cannot assist the mother in the birthing process.
The potential solution to this very serious and frequently fatal problem is that
human babies are born far earlier than expected. Although current human gestation of 38
weeks is not so different from a chimp’s 32 and gorilla’s 37 week gestational period, the
relative development of newborn humans is far behind that of our closest relatives.
Chimps are born with about 80% of their adult brain mass (350 cubic centimeters at birth
compared to 450 ccs at adulthood) while humans are born with only about 25% of their
23
adult brain mass. The altricial state of a human neonate leads many to consider the
newborn more of a fetus developing outside of the womb (Montagu, 1989) and requires a
newborn to be entirely dependent on others, usually the mother, for an extended period of
time. If a parent was not able to devote this significant amount of time and left a
newborn child alone after birth, it would surely die and without a new generation, the
species would soon follow.
The need of a newborn for extended parental, or adult care, continues past
infancy and lasts through at least puberty. With a new generation’s extended period of
need from an older generation (e.g., offspring and parental generations), comes even
greater pressures of societal change and thus, evolution of ancestors social behaviors. A
human juvenile continues to rely on others (usually parental or familial members) for the
basic necessities of food, water and shelter while monkeys and apes are able to provide
sustenance for themselves once they have weaned from their mothers. In comparison to
primate relatives, the prolonged period of time where the young human’s brain grows in
leaps and bounds but requires assistance from adults is certainly unique. This period of
time is so unique, in fact, that humans are the only species of extant primates or early
extinct hominids to have an extra stage of development called childhood. At some point
in our lineage, this crucial stage of brain development housed during the period of
childhood was introduced with the cost of setting up hurdles human parents would need
to overcome in order for their offspring to survive, grow, and eventually reproduce
themselves (Panter-Brick, 2002).
Ultimately, this lengthy time of dependence necessitated a way for parents to
secure enough energetic (food) resources for themselves as well as any offspring still
24
dependent on their care. A division of labor between the sexes, something that evolved
during the Pleistocene era, could have been a solution to the question of how to secure
enough food sources while caring for a helpless child (Pacheco-Cobos et al., 2009). The
sexual division of labor is, in sum, phrased by Lee and DeVore in 1968 as “Man the
Hunter” and by Dahlberg in 1981 as “Woman the Gatherer”. It was the subsistence
method of Homo sapiens up until about 12,000 years ago when farming or pastoral living
became the dominant mode of living. While men went out to hunt large prey, women
foraged nearby for smaller food items. The foraging of nearby items, including grasses
and grains, nuts, fruits and vegetables, and possibly small game could be done while also
caring for smaller children, even carrying the small children on them while gathering.
The division of hunting and gathering set up an ideal solution to overcome the
obstacles of the long periods of helpless children. And it continued to propagate because
women who were better at gathering, and men who were better at hunting, would be able
to provide more for themselves and immediate kin. This division of labor is an early
indicator of sexual strategy employed by men and women to help ensure the survival of
offspring through the difficult period of pregnancy, birth, infancy, and childhood.
Perhaps without any other influence, males would continue to be the hunter
for his monogamous and long-term female partner, and current human sexual strategy
would simply involve finding one, long-term partner to mate and reproduce with for life.
This, of course, is not the case. Although current human populations lean towards
monogamy, there are countless examples that it is not the only sexual strategy. Because
studies find that rates of adultery among married couples range from 26% to 75% (Buss,
25
1993) additional evolutionary changes in our past must also be in play and one likely
candidate is the loss of estrus in females, and the move to concealed ovulation.
Estrus in females occurs in many mammalian species in quite obvious ways.
From the swollen and reddened genitals of the bonobo and baboons to the phermonal
release in many feline species, obvious signs of estrous of the female indicate she is
fertile and provides a clue to males about the female fecund state. The advertisement of
fertility is also a sign that the female is receptive to sexual encounters and thus mating
occurs only when a female is in estrous. Although the mating patterns differ across
species to include single pair bonding, promiscuous bonding, polygamous bonding, or
more complicated mating patterns, the actual mating itself is limited to the time around
estrous when the female is able to conceive (Sillen-Tullberg & Moller, 1993).
In contrast to most primates who exhibit some level of obvious estrous,
human females do not display any known observable signs of ovulation, whether visual
or olfactory, and instead have what is considered concealed ovulation. Concealed
ovulation is simply the loss of any visible, telltale signs of a female’s current fertility,
however the potential for non-direct cues and signals will be discussed later. As a result,
neither a male nor the female herself can know when a woman is ovulating unless other
factors are included (e.g., ovulation kits or tracking the menstrual cycle). Humans
therefore engage in copulation throughout the entirety of a woman’s menstrual cycle and
do not limit this activity to just her periods of fertility. The timing for when loss of
estrus occurred is not yet determined (although many think it was during the Pleistocene
era) and there are a multitude of theories about why it evolved and the impact that loss of
estrus and concealed ovulation had on humans. The majority of the hypotheses,
26
excepting one (Pawlowski’s [1999] theory that concealed ovulation is a by-product of
bipedalism), are implicitly tied into sexual strategies, usually favoring the female by
enhancing her reproductive or survival chances by gaining resources from a male partner.
One early theory that has drawn more support from a recent animal
comparison model is the theory that females who could conceal their ovulation could
confuse the identity of their offspring’s father. This paternal confusion would decrease
the chance of infanticide by males who would be unsure if a child was or was not theirs
(Hrdy, 1979). The theory has gained support from a study of wild Hanuman langurs that
also show concealed ovulation and copulation during non-fertile menstrual phases.
Although the dominant male monopolizes the majority of the female’s time, tests reveal
that non-dominant males are still able to father offspring (Hestermann et al., 2001).
Another result of paternity uncertainty, also demonstrated by the Hanuman langurs, is the
theory that concealed ovulation assists in a female’s ability to cuckhold (i.e. the act of
deceiving a male who thinks he is the father of another male’s child) (Hestermann et al.,
2001). An additional theory that has strong support ties into ideas already presented in
this literature review, namely, that the creation of a human child required an extensive
amount of resources that would be difficult for a mother to obtain on her own. The help
of a partner would vastly improve the chance of successfully raising offspring to
reproductive age and concealed ovulation could help facilitate these strong pair bonds.
Without knowing when any given female is pregnant, a male may actually increase his
chance of reproducing if he continuously mated with one female as opposed to randomly
mating with several females. These continuous matings over an entire cycle would
ensure that at least some of the copulations would occur during a fertile period. It’s
27
further hypothesized in this theory that the continuous matings would strengthen the pair
bond between a male and female (Alexander & Noonan, 1979; Benagiano & Mori, 2009).
Conclusion: Early Pressures Shaped
Differential Sexual Strategies
The differential pressures faced by ancestral males and females, some of
which are presented here, ultimately resulted in a divergence of sexual strategies between
males and females for the simple fact that males and females faced very different
selection pressures. Males and females who were successful in reproducing in the face of
these different sets of selection pressures would have been successful for different
reasons. Females faced the consequence of increasingly difficult births as well as the
need to secure additional resources for offspring during the pregnancy and birth, as well
as during the new developmental period of childhood. Males also faced the pressures of
securing resources for offspring during childhood, however, because females lost the
obvious signs of estrus with concealed ovulation, they also faced a different pressure of
paternal uncertainty. That is, a male could greatly influence the success of his offspring
by giving resources, but there was some risk that he was giving resources to offspring
that were not his. These anthropological theories paint a picture of how early hominoid
species faced selection pressures that favored individuals who adopted roles specific to
their gender’s reproductive challenges, and the answers to the selection pressures are
what moved males and females to the divergence of sexual strategies.
28
Evolutionary Psychology’s Short- vs.
Long-term Strategies
The field of evolutionary psychology (EP) was coined about 20 years ago in
1992 (Barkow, Cosmides & Tooby), making it the newest field to use an evolutionary
lens to understand phenomena within its own field. In particular, evolutionary
psychologists focus on psychological traits exhibited by the evolutionary pressures that
shaped the modern human’s modular brain. At the core of the field, evolutionary
psychologists focus on the “why” of evolved traits while other fields focus on the “how”
of evolved traits by using their model of the Environment of Evolutionary Adpatedness
(EEA; Foley, 1995), which, to reiterate, is a non-specific place and time where ancestors
faced long-standing evolutionary problems to overcome. The importance of this time
rests in the collective niche environment that our ancestors faced, with the solutions to
those problems existing in our current world as behaviors, cognitions, motivations, etc
(Tooby & Cosmides, 1990).
A typical EP question would be, if human females are evolutionary adapted to
be the choosy gender, and human males are evolutionary adapted to be the promiscuous
gender, why do males sometimes seek out long-term relationships (like marriage) and
females sometimes engage in short-term relationships (like one-night stands)? Although
the evidence for inter- and intrasexual selection supports the idea that different
evolutionary pressures led to different sexual strategies in males and females, it does not
adequately explain why males and females engage in these different relationships. Buss
and Schmitt (1993) point out that most research and theories dealing with human mating
exclude short-term mating and focus only on long-term, monogamous relationships, like
29
marriage. However, one must look no further than the line-up of reality TV to know that
any comprehensive examination of human sexual strategy must also include human’s
tendencies to engage in short-term mating. So the question remains, how could both
long-term and short-term relationships have evolved to be a part of both males and
females sexual strategy?
Perhaps not too surprisingly, evidence suggests that males and females could
have used short- and long-term mating to solve different reproductive problems during
our EEA. Males could use short-term mating to increase their reproductive success since
males are not physically hindered during pregnancy and can potentially impregnate an
almost infinite amount of partners. Importantly, a male’s success in increasing their
reproductive success using short-term mating relied on their ability to seek out females
that were both fecund and sexually available. Short-term mating with females who could
not reproduce would fail to create viable offspring and efforts to engage with sexually
unavailable females would result in a loss of time and resources. Males that could use
cues to assess a female’s reproductive fitness and willingness to engage in short-term
mating, such as a female’s age, health and perceived promiscuity, would increase the
likelihood of securing a short-term partner that is more likely to produce offspring.
Further, males benefit if they are not selective in who they choose as short-term mates
and limit their selection criteria to cues of fecundity and sexual availability (Buss &
Schmitt, 1993).
Although short-term mating could confer reproductive advantages to males,
long-term mating could also solve the adaptive problem of reproducing as it would,
among other things, allow a male to devote time and resources to offspring that would in-
30
turn, increase the probability of survival to their reproductive age. Although the number
of offspring is limited to the number of pregnancies one female can have, there is a tradeoff of increased probability of having some of those offspring survive. Additionally, and
as previously described with concealed ovulation, if a male is going to invest time and
resources into offspring, his agreement to enter into a long-term relationship decreases
his chances of cuckoldry. Males selecting long-term partners must still be concerned
with the ability of their partner to conceive, just like in short-term mating, but males in
long-term relationships should be choosier about a long-term partner because the
consequence of a long-term strategy hinders a male’s ability to engage in short-term
mating. In essence, a female should be “worth it” to forego the opportunity for other
short term-mating partners (Buss & Schmitt, 1993).
Like males, females can also solve the adaptive problem of reproduction using
long-term relationships to increase the time and resources available to her offspring.
Females would also gain the protection and resources provided to her during pregnancy
and lactation, a particularly vulnerable time for her. A females choosing a long-term
mate should therefore select a partner who either has resources to give (e.g., time, food,
shelter, protection, etc.), or has some cues or signs as to his potential to accrue these
resources. A female using a long-term relationship strategy that fails to find a mate with
such resources would be putting her and her offspring’s well-being in danger.
Consequently, females should value resources, or the ability to acquire resources, and
should value this more highly than a male’s physical features or cues of fertility (Buss &
Schmitt, 1993).
31
Female’s use of short-term mating as a reproductive strategy has many
proposed advantages and benefits, the most notable being the sexy-son hypothesis
(Fisher, 1930). The sexy-son hypothesis posits that females who prefer the “most
attractive” mate to sire their children will benefit reproductively by having sexy sons who
will inherit their father’s sexy genes and in-turn, will be reproductively successful
themselves (Weatherhead & Robertson, 1979). Like a Fisherian runaway process, the
sexy-son hypothesis is another example of females choosing a secondary sexual trait and
passing on that trait to their sons, and the preference to the trait to their daughters (Mead
& Arnold, 2004). Female’s use of short-term mating could also come with consequences
if she selected the wrong mate as it is the female that ultimately carries the burden of
pregnancy. Additionally, if a female is using short-term mating while engaged in a longterm partnership, she risks losing the resources she would otherwise use for herself and
her offspring. Therefore, a female using short-term mating strategies should seek out
cues in male partners that signal high attractiveness and only seek out those partners
during the most fertile periods of her cycle when pregnancy is most likely (Penton-Voak
& Perrett, 2000).
It is important to clarify two points when understanding these evolved
strategies: 1) males and females not only use these strategies to solve different problems,
but also invest different amounts of time seeking them out; and 2) the evolution of these
strategies did not necessarily create a conscious awareness of their strategic use. Rather,
like any evolved trait, there was natural variation in our ancestors, and those ancestors
who happened to use a short- or long-term strategy at opportune times could have passed
down a preference to engage in the different mating strategies based on both internal and
32
external cues. Males would gain an advantage in seeking out short-term mating more
frequently because it came at a lesser cost, while females should continue to be selective
in their mating. One simplistic research design clearly demonstrates this difference in
short-term mating when college students were approached by confederates of the
opposite sex and asked the question, “Will you sleep with me?”. None of the females
approached said yes to the request while 75% of the males agreed to the short-term
encounter (Clark & Hatfield, 1989).
Research to test how current human populations differentially engage in shortand long-term mating have been done and lend evidence to support the idea that the use
of these strategies is not “strategic” as much as it is an inherent part of our behavior based
on internal and external cues. For example, after determining participants mating
strategies as either being long- or short-term, Confer and colleagues (2010) gave these
participants a choice to view either the face or body of a picture of someone of the
opposite sex. They found that men who were operating under a short-term mating
strategy choose to view the body portion of the picture significantly more than the males
with long-term strategy, who chose to view the face. These findings are expected given
the solutions long- and short-term mating provide. Males who seek out long-term mates
should value higher value cues, like facial beauty which indicates long-term reproductive
value. Males seeking out short-term mating should value cues of fecundity and
availability, which is best assessed by viewing a partner’s body (Confer et al., 2010).
Additional research has added a clue that the preference for these short- and long-term
mating strategies may also be linked to the male’s level of testosterone. In a random
sample of married or unmarried men, unmarried men (or men engaging in short-term
33
relationships) had significantly higher salivary testosterone than married men (Gray et al.,
2002).
Research studying females across the phases of their menstrual cycle not only
indicate that they seek out different cues during different phases, but that they also prefer
seeking out short- or long-term mates at different points as well. In one study regularly
menstruating females, or females who are able to become pregnant, selected the most
masculine face, an indicator of high testosterone, as the most attractive when they were in
their most fertile phase. Females who were not in a fertile phase selected the more
feminine male faces (Penton-Voak & Perrett, 2000). Even in the absence of physical
cues, females who smelled shirts worn by males were likely to rate the men more
attractive overall if they were in their fertile phase (Rantala et al., 2006), if they were in
their fertile phase and the male wearing the shirt had a more symmetrical face indicating
higher genetic quality (Gangestad & Thornhill, 1998; Rikowski & Grammer, 1999), or if
they were in their fertile phase and the male had higher levels of salivary testosterone
(Thornhill et al., 2013). Studies have also found that men perceived as less faithful are
also found to be particularly attractive as a short-term mate when a female is in a fertile
phase (Gangestad et al., 2007). The preference of certain traits in a short-term partner
during the crucial fertile period could have given females an advantage to not only find
good genes to sire sexy sons, but also to find good dads to supply resources to help
support those sexy sons.
Overall, if the preference to engage in short- or long-term mating with
particular partners based on external (like the masculine facial features driven by high
levels of testosterone) or internal cues (like preferring such mates during fertile periods)
34
had a genetic component that was inherited by future generations, we could expect the
current population to also engage in such behaviors. Because many of these strategies
are driven by unknown cues, like attractiveness to particular parts of the body, or
preference for individuals with higher levels of testosterone during the concealed time of
fertility, the engagement of the strategies could potentially be directed by some
genetically inherited traits.
2D:4D Ratio, Hormones, and Sexual Strategy
The amount of testosterone is one particular cue that is an excellent contender
as it is a key player in both male and female short-term strategies. It has been shown to
be correlated to males’ marital status (Gray et al., 2002) as well as an indirect measure of
male preference by females, even without visual cues (Gangestad & Thornhill, 1998;
Rikowski & Grammer, 1999; Rantala et al., 2006; Thornhill et al., 2013).
The ratio of the 2nd to 4th finger (the 2D:4D ratio) has been shown to be an
indicator of the amount of androgens present during fetal development in the uterus. The
relationship between 2D:4D ratio and intrauterine androgen presence is inversely related.
Higher levels of prenatal androgen is related to a smaller, more “masculine” 2D:4D ratio
while lower levels of prenatal androgen is related to a larger, more “feminine” 2D:4D
ratio (Lutchmaya et al., 2004). Evidence of 2D:4D sexual dimorphism was found early
in the scientific literature (George, 1930) and countless studies since have reported a
large body of significant correlations both in human as well as animal studies.
In animal studies, fetal environments can be experimentally controlled and
manipulated. The ability to create experimental groups allows for the ability to predict if
35
prenatal hormones cause the effect of digit ratios as opposed to merely correlating digit
ratio with prenatal androgens. Pheasant eggs that were injected with testosterone to
increase the concentration during embryonic development showed adult sexual
dimorphism in the 2D:3D ratio while control adults did not, but only in the female
offspring (Romano et al., 2005). The results indicate that an increase in testosterone does
cause differences in digit ratio. A similar study was conducted by injecting pregnant rats
with a single dose of testosterone during the third trimester, a critical time for
differentiation of male and female rats. The control group was injected on the same day
with a placebo. The resulting experimental female progeny showed more similar patterns
of 2D:4D and other behaviors with the male controls than with the female controls, again
indicating that the effects of prenatal hormones, specifically testosterone, plays a role in
determining later adult behaviors and physical features.
It is much more difficult to create experimental conditions in human fetal
development to determine cause and effect. However several studies do support the
relationship between 2D:4D and prenatal androgens. In a study of deceased human
fetuses, there was a difference in 2D:4D between males and females with males 2D:4D
being significantly smaller, or more masculine (Galis et al., 2010). Another study found
that the amount of fetal testosterone was correlated to 2D:4D at age 2 in the expected,
inverse direction (Lutchmaya et al., 2004). 2D:4D was also shown to be consistent over
time in a sample of Jamaican children between a four year span (Trivers et al., 2006).
Studies of 2D:4D in adults have also revealed many significant relationships
between 2D:4D and sexual behavior or characteristics. Men show an inverse relationship
between 2D:4D and sperm number resulting in men with more masculine (smaller)
36
2D:4D ratios having higher sperm counts (Manning et al., 1998). Additionally, the
number of sexual partners a male has is also inversely related to their 2D:4D. Even after
controlling for age, education, occupation and relationship status, men with more
masculine 2D:4D ratio have significantly more sexual partners (Hönekopp et al., 2006).
Behaviorally speaking, men with masculine 2D:4D ratios reject unfair offers in
monetarily based ultimatum games except when they are first primed with sexual images.
In those cases, men with lower, more masculine 2D:4D ratios are more likely to accept
unfair offers (Van den Bergh & Dewitte, 2006), which is a potential indicator of their
preference of mating opportunities over short term monetary gains. Because androgen is
a precursor to testosterone, and because there is evidence of a correlation between some
sexual behaviors and 2D:4D, it is plausible that a person’s 2D:4D could be related to their
sexual strategy, particularly when it comes to short-term mating selections.
Research Aims
Taken together, the reviewed literature poses a strong argument that
evolutionary pressures our ancestors faced in our EEA has shaped sexual strategies that
differ both between males and females, as well as within males and females. Like
countless other species, human females have evolved to be the more choosy gender given
the increased risks they face with reproduction (Trivers, 1972), and can benefit by
choosing mates who have sufficient access to resources (Buss & Schmitt, 1993).
However, females do engage in short-term mating and this can increase their
reproductive fitness if they use cues to choose a short-term mate that will pass on the
genes that will be desirable in future generations (Gangestad et al., 2007). Males strategy
37
includes competing with other males for access to sexually available and fertile females,
but they could increase their reproductive potential if they are attracted to females that are
able to reproduce at the moment (for short-term mating) or are high value mates with
reproductive potential (for long-term mating). Hormones, particularly testosterone, play
a large role in promoting these behaviors, cognitions, and attractions, despite that fact that
the mechanism for the roles are hidden from us. In sum, the outcome from these hidden
mechanisms from generations upon generations of evolutionary pressures are a set of
variable motivations making up our species sexual strategy.
As such, this investigation seeks to test several research questions to test how
these pressures have shaped our current sexual strategy, how they differ between males
and females, and how they vary within each gender based on biological factors, such as
2D:4D. Research question 1 will test the idea that males’ and females’ differential sexual
strategy will result in a difference in the frequency of sexual partners and sexual
encounters with the hypothesis that females are choosier and males more promiscuous.
In addition to asking participants how many actual sexual encounters and sexual partners
they have had, they are asked how many they would like to have, or their “ideal” number
of sexual partners and frequency. This distinction is critical as the frequency of actual
sexual partners and encounters assess actual behavioral differences, while questions
about the ideal frequency better assess underlying motivations, providing a measure of
sexual selection strategies independent of behavioral opportunities (e.g., wealth,
proximity to females, health, etc.) to express those motivations. Research question 2 asks
whether differentiation between the sexes and degree of differentiation among the sexes
is predictable and related to hormonal differences in utero. Although direct measures of
38
hormones were not assessed, the indirect marker of prenatal hormones measuring
participants 2D:4D (the ratio of the second to fourth digit) makes an excellent
substitution as it has been robustly shown to distinguish between genders as well as
differentiate between behaviors within gender. Finally, Research question 3 asks whether
cognitive, behavioral and biological sexual strategy differences result in the culmination
of sexual “norms” for males and females. It will test if males and females confirm to a
standard “norm” of stereotypical masculine and feminine traits. Further, the study will
look at what that norm may look like, and, in particular, how that relates to biological
indicators of masculinity (i.e. 2D:4D ratio).
CHAPTER III
METHODOLOGY
Participants
One hundred ninety-seven participants were sampled from undergraduate
psychology courses at a midsized Western university. They received extra credit in their
psychology courses for their participation in this study. Fifty-five participants were male
(27.9%), 137 were female (69.5%) and five (2.5%) did not provide their gender.
Participants were primarily white (72.1%) with an average age of 21.8 (SD = 3.6).
Measures
Sexual Behavior and Attitude Survey
The Sexual Behavior and Attitude Survey (SBAS, see appendix A) was
created to assess the frequency of, and attitudes towards, sexual behavior. Participants
were asked to report how often they engaged in various sexual activities either in the
previous month or the previous six months (e.g., number of sexual encounters in the last
month, number of orgasms in the last month, etc.). These reports included the actual
frequency of the sexual behavior and the ideal (i.e., desired) frequency of these
behaviors. For example, participants were asked to report on both their actual number of
sexual partners and their ideal number of sexual partners in the past six months.
Participants also responded to how much they agreed and disagreed to a series of
39
40
statements about ideals and behaviors relating to monogamy and promiscuity on a Likert
scale ranging from 1, (“Strongly Disagree”) to 5, (“Strongly Agree”). For example, items
included “I find myself sexually attracted to many people” or, “I prefer a life where I
settle down with one person to whom I am completely committed.”
Participants were asked to provide their actual and ideal frequencies of sexual
behaviors for six different sexual behaviors (12 items total) and responses were log
transformed to account for skewness and kurtosis (Bland & Altman, 1996). Although log
transformed scores were used in statistical analyses, raw mean values are reported for
interpretability (Manikandan, 2010).
Demographic questionnaire
Basic demographic information regarding participant’s age, gender,
relationship status, ethnicity, and religion were assessed. Participants were also asked to
report on their sexual orientation on a 5-point Likert scale ranging from 1, (“Exclusively
Heterosexual”) to 5, (“Exclusively Homosexual”). This scale is a modified version of
Kinsey’s (1948) 7-point Likert scale for sexual orientation. See Appendix B to view this
questionnaire in this entirety.
Bem’s Sex Role Inventory – Short Version
The Bem’s Sex Role Inventory – Short Version (S-BSRI; Bem, 1974, 1981) is
designed to measure “masculinity”, “femininity”, “androgyny” (i.e., having both
masculine and feminine features), or “undifferentiated” personality features based on
socially desirable and stereo-typical gender-specific personality traits. The short version
of the measure contains 30 of the original 60 single word items in which participants are
instructed to rate themselves on a 7-point Likert scale, ranging from 1, (“Never or almost
41
never true”) to 7, (“Always or almost always true”). Of these 30 items, ten comprise the
masculine score, ten comprise the feminine score, and ten items are neutral. Responses
to each of the ten masculine and ten feminine items are averaged and assigned, by Bem’s
measurement, a standardized score based on this average value. The measure allows
independence of scores on both the masculinity and femininity scale – a score on one
scale has no bearing on the score of the other scale. The original psychometric analysis
reported alphas of .86 and .80 for the masculine and feminine scores respectively (Bem,
1974). Test-retest scores were calculated for a subset of participants who again took the
BSRI. Product-moment correlations analysis revealed .90 for both the masculine and
feminine scale (Bem, 1974).
More recent psychometric analyses have shown that the questionnaires’ scales
are robust despite the passing of nearly four decades since the creation of the scale. Holt
and Ellis (1998) found alphas of .95 and .92 for the masculine and feminine scale
respectively. The present sample found alphas of .80 and .87 for the masculine and
feminine scales respectively.
Biological indicator: 2D:4D
To measure participant’s second to fourth digit ratio, a digital photograph was
taken of the inside of the participant’s outstretched hands. The length of participants’
index (second digit or “2D”) and ring fingers (fourth digit or “4D”), starting from the
ventral proximal crease to the tip of the finger (Manning, 2002) was measured from these
digital photographs using caliper software (Iconico, 2006). Participants were asked to
remove or pull rings off of their ventral proximal crease to allow for more accurate
measurement. A ratio was calculated by dividing the length of the index finger by the
42
length of the ring finger (e.g. 2D/4D) for each hand separately. The current sample had a
right hand 2D:4D that ranged from .90 to 1.05 (X = .97, SD = .03) and left hand 2D:4D
that ranged from .87 to 1.08 (X = .96, SD = .04).
2D:4D has been putatively related to the concentrations of prenatal gonadal
hormones in vitro (e.g., Lutchmaya et al., 2004; Manning, 2002; Ökten et al., 2002) and
is inversely related to these prenatal hormones such that a lower ratio (smaller second
digit than fourth digit) is thought to correspond with higher levels of prenatal testosterone
than estrogen levels. As such, males typically have lower 2D:4Ds than woman. Caliper
measurement from digital photographs were tested to be one of the most reliable means
to measure and calculate 2D:4D (Kemper & Schwerdtfeger, 2009) when compared with
other means to collect 2D:4D (e.g., in person caliper measurement or measurement with a
plastic ruler).
Procedures
Participation in the study included answering four self-report
questionnaires and having two digital photographs of their palms taken. Participants
arrived to the study session location (a classroom within the Social Science College) and
completed the study assessment in groups of 1-20 participants. Upon arrival to the study
session location, participants were instructed to choose a manila envelope located at the
front of the classroom. The envelope had a unique identification code listed on the front
and contained the four questionnaires, in differing order, with the same unique
identification code on each questionnaire. On the front of each envelope, a pre-fabricated
hand placement template was attached which also listed the unique ID. Participants were
43
then instructed to choose and take a seat at an individual desk, open the manila envelope,
and complete the four questionnaires. After completion of the questionnaires,
participants were instructed to remove all jewelry on the hands or to pull jewelry up to
their middle knuckle so that an unobstructed photograph of their palms, including the
creases of each finger, could be taken. Participants were asked to place their hands, palm
side up on top of the pre-fabricated hand placement template, in the designated area for
their left and right hand so that their unique ID could also be included in the picture. The
digital photographs were taken at a single station where a tripod and black poster board
were assembled to ensure anonymity (e.g., the digital camera operator could not see the
participants face, their hands, or the resulting digital photograph).
After completing the questionnaires and having a photograph of their hands
taken, participants were instructed to put all questionnaires within their manila envelope
and place the manila envelope in a sealed box. At that time, they were given a piece of
paper certifying they had completed the experiment which could be turned in for extra
credit and a debriefing sheet.
Statistical Analysis
To test and address the three hypotheses, several analyses were run using
SPSS (version 19). Analyses include correlations to examine relationships between
variables, correlations with gender split by males and females to control for the effect of
gender while examining the relationships between variables, and independent and paired
samples t-tests to identify mean differences between groups. Levene’s test of
homogeneity of variance was analyzed for all t-tests, and a comparison of variance
44
assumed and variance not assumed were compared for all Levene values resulting in p <
0.05. In all cases where Levene’s test resulted in a p < 0.05, there were no differences
between the significance of the independent samples t-test; both were either significant or
not significant. For that reason variances assumed was used for all independent samples
t-test.
CHAPTER IV
FINDINGS AND RESULTS
Testing Research Question 1 – Do Males and
Females Differ on their Sexual Behaviors?
Independent samples t-tests were conducted to test for the similarities and
differences between males’ (as a group) and females’ (as a group) reports on actual and
ideal sexual behaviors. First, differences in ACTUAL sexual behaviors when those
sexual behaviors involved a partner were tested. Because males and females, generally,
should not report higher rates of paired sexual activity comparatively, it was necessary to
test any reporting error between sexes. Independent samples t-tests then compared the
difference between males and females on their IDEAL frequency of sexual behaviors. As
expected, males and females do NOT differ significantly in terms of their reported, actual
number of sexual partners in the past six months (males = 1.4 sexual partners, females =
1.5 sexual partners, t(190) = -0.37, p = .71) or the actual number of sexual encounters in
the past month (males = 8.4 sexual encounters, females = 8.0 sexual encounters, t(190) =
-1.22, p = .23). When it came to males’ and females’ ideal sexual behaviors, the
difference in desired number of sexual partners over the past six months was significantly
different (males = 2.1 sexual partners, females = 1.5 sexual partners, t(190) = 2.81, p =
.005). Examining these mean differences in greater detail highlights the differences
45
46
between males and females desired number of sexual partners. Females reported only
wanting one, or zero sexual partners (75.2% compared with males 56.4%) while male
reports had greater spread and were slightly more favorable to higher numbers of sexual
partners (e.g., in comparison to females, males who report wanting 2 or fewer sexual
partners account for 63.6%; 3 or fewer sexual partners account for 81.8%). However,
males and females did not differ significantly on their desired number of sexual
encounters over the past month (males = 19.6 sexual encounters, females = 12.9 sexual
encounters, t(188) = 1.87, p = .06). These findings indicate collectively that the key
difference between males and females is not how much sex they are having or would like
to have, rather, the difference in the number of different partners they would like to have
with males preferring to have a greater variety of sexual partners compared with females
(see Table 1).
To test whether ideal and actual sexual behaviors differed within each gender
(e.g., do males’ ideal sexual encounters differ significantly from their actual sexual
encounters), the ideal values for sexual partners and sexual encounters were compared to
the actual values by paired samples t-tests. These tests will help understand if males and
females differ in their desire for sexual activity and to what degree. A positive result
indicates a desire to have fewer sexual partners or encounters, a negative result indicates
a desire to have more sexual partners or encounters and a result around zero indicates the
ideal and actual frequency are the same (i.e., having as many sexual partners or
encounters as one prefers). These pair-wise tests were conducted independently within
males and females. Both males and females significantly differed in their actual versus
ideal sexual encounters, with males reporting a difference of -11.50 (t(53) = -6.98, p
47
Table 1.
Independent samples t-test of males’ vs. females’ relationship preferences and actual
and ideal sexual behaviors
Males
Females
N
Mean±SD
N
Mean±SD
t
Sig.
Actual
55
1.45±1.17
137
1.54±1.42
-0.37
0.71
Ideal
55
2.15±1.85
137
1.49±1.47
2.75
0.01
Actual
55
8.41±15.35
137
8.00±11.09
-1.14
0.26
Ideal
55
19.65±23.32
137
12.94±14.95
1.87
0.06
55
4.13±1.04
137
4.45±0.91
-2.15
0.03
I find myself sexually attracted
to many people
55
3.85±1.01
137
3.33±1.11
3.18
<0.00
Wanting sex with many people
is normal
55
3.77±0.91
137
3.08±1.05
4.29
<0.00
55
4.42±0.83
137
4.45±0.79
-0.21
0.83
Sexual Partners in past 6 months
Sexual Encounters in past month
Relationship Preference
I prefer having one long-term
sexual partner over many
short-term sexual partners
I prefer a life where I settle
down with one person to whom
I am completely committed
NOTE – Raw values are presented for means and standard deviations for readability.
Log-transformed values are used for statistical analysis.
48
<.001) and females reporting a difference of -4.92 (t(135) = -4.88, p <.001). Again, this
negative difference indicates that both males and females reported wanting more sexual
encounters over the past month than they actually had with males reporting to want, an
average 11.5 more sexual encounters and females 4.92. Males significantly differed in
their reports of ideal versus actual sexual partners by an average rate of -0.70 (t(54) = 3.00, p = 0.004) indicating that males wanted, on average, 0.70 more sexual partners in
the previous six month period. Unlike males, females did not significantly differ on their
reported ideal versus actual sexual partners; the difference between females’ ideal and
actual sexual partners over the past six months was only 0.05 (t(136) = 0.27, p = 0.79).
In summary, the actual and ideal sexual behavior findings indicate that males reported
wanting significantly more sexual encounters and partners while females also wanted
significantly more sexual encounters but were “content” with the number of sexual
partners (see Table 2).
Independent samples t-tests were run on a series of questions to test if males
and females differ in the types of relationships they look for (long- vs. short-term), and if
they prefer monogamous or promiscuous sexual styles. These analyses will further test
if, and how, males and females differ in their sexual behavior by specifically testing if
they have significant differences in their preferences for relationship types. When asked
to answer the statements, “I find myself sexually attracted to many people” and “Wanting
sex with many people is normal”, males endorsed the statements significantly more
highly than females (t(190) = 3.06, p = 0.003 and t(190) = 4.29, p = <0.001 respectively).
This indicates that males are more inclined towards more promiscuous thinking in regard
to the types of sexual relationships they would like to have when compared to females.
49
Table 2.
Paired samples t-test comparing males’ and females’ actual and ideal sexual behaviors
Actual
Ideal
N
Mean±SD
N
Mean±SD
Mean
Diff±SD
t
Sig.
Sexual Partners
55
1.45±1.17
55
2.15±1.85
-0.70±1.93
-3.00
<0.01
Sexual Encounters
54
8.15±15.37
54
19.65±23.32
-11.50±18.18
-6.98
<0.01
Sexual Partners
137
1.54±1.42
137
1.49±1.47
0.05±1.33
0.27
0.79
Sexual Encounters
136
8.02±11.13
136
12.94±14.95
-4.92±9.61
-4.88
<0.01
Males
Females
NOTE – Raw values are presented for means and standard deviations for readability.
Log-transformed values are used for statistical analysis.
Conversely, when asked to answer how much participant’s agreed with the statement, “I
prefer having one long-term sexual partner over many short-term sexual partners”,
females endorsed the statement significantly more than males (t(190) = -2.15, p = 0.03),
indicating females prefer long-term relationships more than males (see Table 1).
50
Testing Research Question 2 – Does 2D:4D
Ratio Serve as a Biological Marker for
Differential Prenatal Hormones and
Sexual Behavior?
Participant’s 2D:4D was analyzed to see if it would predict sexual behaviors
within the sexes. However, because 2D:4D is hypothesized to be a result from prenatal
androgens (testosterone), the calculated 2D:4D was first tested to see if it would differ
significantly between males and females. Because males have significantly more
prenatal testosterone exposure than females, and because 2D:4D is hypothesized to vary
based on the presence of testosterone during fetal development, we should expect to see
2D:4D to differentiate biologically between males and females. An independent samples
t-test comparing males and females 2D:4D found significant differences in the expected
direction, but only for the left hand (not significant in the right hand although average
2D:4D values are in the expected direction). Males’ average 2D:4D on the left hand was
calculated at 0.948 (SD = 0.031) versus 0.963 (SD = 0.037) in females (t(190) = -2.64, p
= 0.009; d = -0.43). Although the differences reported are small, the effect size for this
analysis (d = 0.43) was found to exceed Cohen’s (1988) convention for a small effect (r =
0.21).
Correlations were conducted within each gender to test the relationship
between males’ and females’ actual and ideal sexual behaviors with their 2D:4D. The
only sexual behavior that was significantly related to 2D:4D in the anticipated direction
was the actual number of sexual encounters over the past month and only for males
(males r = -0.29, p =0.03; females r = 0.03, p = 0.77). Further, when testing the
difference between these two independent correlations, males’ correlation was
51
significantly different than females’ (z = 2.01, p = 0.044). No other test of difference
between two independent correlations were significant. That is, as males’ 2D:4D
decreases (becomes more “masculine”), their actual number of sexual encounters
increases. Although not hypothesized, males had a significant positive relationship
between the actual and ideal number of times they viewed pornography in the past month
(actual r = 0.31, p = 0.020; ideal r = 0.30, p = 0.028). This significant positive
relationship can be interpreted to mean that the greater a males’ 2D:4D (becomes more
“feminine”), the more frequently they view pornography (see Table 3).
Table 3.
Correlations of males and females left hand 2D:4D with actual and ideal sexual
behaviors
Actual
Ideal
Males
Females
Males
Females
Sexual partners in past 6 months
-0.03
-0.01
0.07
0.00
Number of masturbations in past month
0.20
0.06
0.21
0.07
Sexual encounters in the past month
-0.29*
0.03
-0.13
0.07
Number of times viewed pornography in past
month
0.31*
0.08
0.30*
0.11
Number of images per pornography session
-0.09
0.08
0.07
0.12
Number of orgasms in past month
-0.06
0.04
-0.07
0.00
* < 0.05.
52
Correlations were run with genders combined and genders split on the
promiscuous/monogamous and short-term/long-term relationship variables with 2D:4D
ratio to test for a relationship between a person’s relationship preference and levels of
prenatal androgen. Because females who secured a long-term or monogamous
relationship could potentially increase chances of survival through resources from a
partner, females should score more highly on the monogamous and long-term
relationship scores. Males, based on their sexual strategy, should lean towards
promiscuous and short-term relationships. Further, if prenatal androgen’s have an effect
on adult sexual behavior, more “masculine” 2D:4D ratios should relate with promiscuous
ideals while “feminine” 2D:4D ratios with monogamous ideals, regardless of gender.
With genders combined, there was a significant correlation between the left hand 2D:4D
ratio and endorsement on the statement, “I prefer having one long-term sexual partner
over many short-term sexual partners” (r = 0.18, p =0.01) so that as a participant’s 2D:4D
ratio gets larger (i.e., becomes more feminine), their preference for long-term relationship
also goes up. When genders were split, the significant, positive relationship still held for
males (r = 0.27, p =0.04), but not for females. No other relationships were found to be
significant, nor were the tests of difference between the two independent male and female
samples significant (see Table 4).
53
Table 4.
Independent samples t-test of males’ vs. females’ 2D:4D ratio and relationship
preference
Left Hand Ratio
Males
I prefer having one
long-term sexual
partner over many
short-term partners
Right Hand Ratio
Females Combined Males Females Combined
0.27*
0.11
0.18*
0.23
0.06
0.13
I find myself sexually
attracted to many
people
-0.04
0.07
0.00
0.02
0.02
0.00
Wanting sex with
many people is normal
0.01
0.03
-0.03
0.12
0.08
0.06
0.13
0.00
0.04
0.20
0.00
0.07
I prefer a life where I
settle down with one
person to whom I am
completely committed
*< 0.05
Testing Research Question 3 – Are there
“Norms” Between Males and Females
Shaped by Culture that Account for
their Sexual Strategy?
Participant’s score on the BSRI was analyzed to test to see if 1) males and
females differ significantly on the BSRI and 2) if these scores have any biological basis.
To test if there are significant differences on the BSRI between males and females, an
independent samples t-test was run comparing the standard feminine and masculine
54
scores between the two genders. There were no significant difference between males and
females on the standard masculine score (t(190) = -0.90, p = 0.37) however, there was a
significant difference between males and females on the standard feminine score where
females scored significantly higher (51.64 for females compared with 47.36 for males,
t(190) = -2.49, p = 0.01) (see Table 5).
Table 5.
Independent samples t-test of males’ vs. females’ BSRI standard and individual item
scores
Males
Females
N
Mean±SD
N
Mean±SD
t
Sig.
Standard Masculine Score
55
47.85±9.12
137
49.25±10.05
-0.90
0.37
Standard Feminine Score
55
47.36±12.27
137
51.64±10.11
-2.49
0.01
NOTE – Significant items bolded for readability.
To test if there are any biological bases for differences on the BSRI, bivariate
correlations were run comparing 2D:4D and the BSRI masculine and feminine standard
scores on the BSRI. The first set of correlations had males and females combined and the
second set of correlations had males and females separated to account for differences
between gender. There were no significant correlations with left hand 2D:4D ratio and
feminine or masculine standard scores when males and females were combined or for
males and females when gender was split (see Table 6).
55
Table 6.
Left and right hand 2D:4D correlated with BSRI standard and individual item scores for
males, females and males and females combined
Left Hand Ratio
Males
Females Combined
Right Hand Ratio
Males
Females Combined
Standard Masculine
Score
-0.11
-0.04
-0.04
-0.26
-0.02
-0.08
Standard Feminine
Score
0.21
-0.05
0.06
0.23
-0.11
0.03
*< 0.05; **< 0.01
CHAPTER V
CONCLUSIONS AND RECCOMMENDATIONS
Several findings from the study support the theory that males and females
approach sexuality differentially. Specifically, the study touched on behavioral,
cognitive, and biological differences. It is important to begin with the findings that both
males and females in the sample reported having the same number of sexual partners and
sexual experiences. The fact that males’ and females’ reports of actual sexual behaviors
are approximately equal and are not significantly different is expected given the
approximate equal population of males and females in the college setting and that sexual
activity likely is happening within that population. Given the likelihood that males and
females in this sample are seeking out sexual partners that are homologous to the
sample’s demographics, one can infer that the reported frequency of sexual activity is
realistic and that one gender is not over- or under-reporting their sexual activity
frequencies. In contrast to actual sexual activity, males reported wanting significantly
more sexual partners and sexual encounters while females were content with the number
of partners they had but did report wanting more sexual encounters with those partners.
These findings fit within the proposed framework of sexual strategy that expects males to
engage in behaviors or have motivations that tend to increase their chances of
reproductive success by having a high frequency of sexual encounters with a higher
frequency of sexual partners; females are expected to prefer those sexual encounters with
relatively fewer partners, however an increase frequency of sexual encounters could
56
57
potentially cement the bonds of a long-term relationship (Bateson, 1983). Indeed,
females reported only desiring 1.5 partners, on average, over the past six months
(compared to 2.1 for males) with the majority of females (i.e., 75.2%) reporting only
wanting one or no partners. Additionally, males reported a stronger agreement for
promiscuous types of relationships as well as a preference for short-term relationships
compared with females. These findings provide further support for Darwin and other
researchers in sexual research who contend that males would like to be more
promiscuous with more partners while females are more selective about their partners, or
choosier.
While males have a significant desire for more sexual partners, encounters,
and promiscuous and short-term relationships, it is important to note that the impetus for
these sexual acts are not likely because they are cognitively aware that it will increase
their sexual reproductive success. In turn, females most likely are not being choosy or
selective about their sexual partners as an intentional decision and strategy to invest their
few viable zygotes into highly desirable genes. Instead, it is more likely that evolved
differences in reproductive strategies resulted in evolved, underlying motivational
differences between males and females that, on average, resulted in reproductive rewards
of each of these strategies (i.e., reproduction is more successful for “promiscuous” males
and “choosy” females) (Bateson, 1983; Buss & Schmitt, 1993).
If these are indeed evolved tendencies, there must be biological mechanisms
that drive these sex differences in reproductive strategies that could have been passed on
across generations. The current study investigated one possible biological mechanisms
(prenatal testosterone) and whether differential prenatal testosterone (measured using
58
2D:4D) exposure might predict differences in sexual behavior. As expected, 2D:4D did
correctly distinguish the females in our group from the males. Males had a more
“masculine” right hand 2D:4D ratio compared with females, suggesting a higher rate of
prenatal testosterone present during male fetal development. Results suggested that
males with a more masculine 2D:4D ratio also reported higher frequencies of actual
sexual encounters as well as endorsing a higher preference of short-term relationships.
This finding is consistent with the hypothesis that hormones present in utero could have
long lasting effects on later, sexual adult behaviors (Goldstein et al., 2013). Additionally,
this significant results were in the hypothesized direction based on the historical evidence
of evolved sexual strategy that higher testosterone would predict males seeking out more
sexual encounters and preferring short-term relationships. The ability for in utero
hormones to change later sexual behavior could be one potential vector responsible for
evolved changes in sexual strategy. Interestingly, other correlational results of 2D:4D
ratio and sexual behaviors were not significant for females, or for other sexual behaviors
in males (e.g., their ideal number of sexual encounters, or the number of actual or ideal
sexual partners).
The lack of a relationship between a female’s 2D:4D and her sexual behaviors
in the current study may be because prenatal testosterone is not the mechanism that
evolved to make females choosier. While testosterone is present in females, there are
several other hormones that are responsible for female’s growth and development, such
as the ovarian hormones progesterone and estrogen. Further, because males and females
do appear to have different sexual strategies, it is unlikely that these different strategies
arose through selective pressures on the same biological mechanisms. It is therefore
59
possible, and likely, that there was a different set of biological mechanisms that drove the
“choosy” trait for females that were not included in the current study. Alternatively,
differences may be due to cultural norm differences per se, suggesting that enculturation
plays a bigger role in both underlying motivations and manifest behavior than is assumed.
Additionally, the current study could have included additional and more appropriate selfreport measures to test the sexual strategy of females more accurately (e.g., measures
relating to mate selection, monogamy, etc.).
The BSRI was used to test the hypotheses that males and females endorse
norms of femininity and masculinity differentially; both differentially between the
genders and within the genders based on 2D:4D. However, the results from the BSRI
analysis could not support this hypothesis. One potential explanation for the lack of a
relationship could be due to the unreliability of the BSRI. Specifically, results using the
BSRI scale seemed unreliable in the present sample for two reasons. First, while females
did score higher than males at mean level differences for the feminine scale, there was no
significant difference between males and females on the masculine scale. Secondly,
females scored higher on 25 of the 30 items compared to males with more than half being
a significant difference (i.e., 13 of the 25 where p ≤ 0.05). Again, a higher score on any
item indicates higher endorsement and agreement that the item describes the participant.
Of these 13 items, six were “feminine” items, three were “masculine” items, and four
were neutral items. Although the item level analysis is anecdotal and not meant for
individual analysis, it indicates that females endorsed their agreement on the BSRI, even
on the masculine items. Further inconsistencies arose when taking into account the
relationship between 2D:4D, BSRI scores, and gender. While several individual items on
60
the BSRI were significantly correlated with 2D:4D for males in the sample, neither the
masculine or feminine standard scores reached significance. Unfortunately, most of these
significant relationships were for the right hand 2D:4D only. Because previous analysis
in this study using the right hand 2D:4D failed to distinguish between males and females,
significant results with the right hand 2D:4D ratio should be considered with caution.
The lack of any cohesive patterns using the BSRI does not allow for the interpretation of
results.
Taken together, the findings from the first two hypotheses support the idea
that males and females have differential sexual strategies. Importantly, the findings
support the current argument that these differential sexual strategies (which include a
combination of behaviors and cognitions) may have evolved to best support that gender’s
chance of reproductive success (Bateson, 1983; Buss & Schmitt, 1993).
Limitations
Research using 2D:4D ratio has some critics as numerous other studies have
found no relation to 2D:4D ratio or in fact, have found conflicting results. However,
recent literature indicates 2D:4D ratio measurements of the hand are actually only a
proxy for the “real” 2D:4D ratio which is the length of the bone in each digit (Xi et al.,
2014). Perhaps the mix of results is an outcome of both the small variability of finger
lengths between individuals (i.e., 0.947 in males vs. 0.965 in females [Bailey & Hurd,
2005]) and the less than reliable means of measuring 2D:4D ratio (e.g., tracings of
fingers, participant’s self-report, etc.). Further research, including refined methods for
measuring 2D:4D ratio and continuing lines of animal research, will further the use of
61
2D:4D ratio as a measure to better understand if, and in what ways, prenatal androgens
cause lasting effects on both cognitions and behaviors throughout life. The current
study’s use of digital photographs to measure 2D:4D ratio are therefore somewhat limited
in the extent to which we can measure the real 2D:4D ratio and future studies should
implement, if possible, measurements such as X-rays that will give the bone, instead of
finger, 2D:4D ratio. More accurate measures of bone 2D:4D ratio would likely result in
additional findings for males (but perhaps not females as discussed previously) as well as
increase the strength of the relationship of variables already found to be related.
However, since the current study could still significantly distinguish between sex, it is at
least a sensitive enough measure to have some predictable power.
Like other studies, all of the measures, except the calculation of participant’s
2D:4D ratio, were self-report. Self-report measures, while easy and convenient, have
disadvantages, namely, that the responses cannot be confirmed and are dependent on the
participant’s willingness to be honest and their ability to remember past details correctly.
Additionally, researchers have indicated that participants can be particularly sensitive and
hesitant to respond honestly, or at all to questions about sexuality (Buss & Schmitt,
1993). However, many steps were taken to ensure participant confidentiality (e.g.,
random ID assignment, participant’s completing the measures in group instead of
individual setting, and submitting their responses within a sealed envelope) and these
steps were stressed to the participants during the informed consent process. As
participation in the study was completed by maximizing confidentiality, participants were
encouraged to be as honest as they possibly could, to take time to report the most
62
accurate responses, and to skip questions they either could not remember, or did not feel
comfortable reporting on.
The current sample of college aged participants within the psychology
department, while common in this field of research, has several limitations. Although the
sample had a sufficient number of participants, it included twice as many females than
males (although appropriate means, such as testing for equal variances were
implemented). The study was also limited to a particular developmental age period given
that all participants were college-aged. Finally, because all participants are college-aged
and enrolled in the university, the results may have limited generalizability. Although the
sample is most likely a good representation of adult humans, future research should
include a broader sample of participants and include 1) more males; 2) varied
developmental ages; and 3) diversity of education.
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APPENDIX A
Sexual Behavior and Attitude Survey
Below are a series of questions concerning your behaviors and attitudes about sex. Your honesty
is important. Consider each question carefully and be as accurate as possible in your responses.
For each pair of items below you will be asked sexually related questions concerning
your actual behavior and your ideal (preferred) behavior (assume no negative
consequences). Please be accurate and be honest.
1. Actual number of different sexual partners over the past 6 months: ________
2. Ideal number of different sexual partners over the past 6 months: _________
3. Actual number of masturbation sessions over the past month: _______
4. Ideal number of masturbation sessions over the past month: _________
5. Actual number of sexual encounters (with a partner) over the past month: _______
6. Ideal number of sexual encounters (with a partner) over the past month: ________
7. Actual number of times you viewed pornography over the past month: ________
8. Ideal number of times you viewed pornography over the past month: ________
9. Actual number of different images viewed during a single session of pornography
viewing: _______
10. Ideal number of different images viewed during a single session of pornography
viewing: _______
11. Actual number of orgasms over the past month: ________
12. Ideal number of orgasms over the past month: ________
Please circle your response below each of the following questions.
13. I prefer having one long-term sexual partner over many short-term sexual partners.
Strongly Disagree Disagree Neither Agree nor Disagree Agree Strongly Agree
14. I find myself sexually attracted to many people.
Strongly Disagree Disagree Neither Agree nor Disagree Agree Strongly Agree
15. Wanting sex with many people is normal.
Strongly Disagree Disagree Neither Agree nor Disagree Agree Strongly Agree
16. I prefer a life where I settle down with one person to whom I am completely
committed.
Strongly Disagree Disagree Neither Agree nor Disagree Agree Strongly Agree
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APPENDIX B
Demographic Information
Below are a series of questions concerning your behaviors and attitudes about sex. Your
honesty is important. Consider each question carefully and be as accurate as possible in
your responses.
General Information:
Age: ______
Sex: ______
Relationship Status: ________________________
Sexual orientation (circle one number):
1
2
Exclusively Heterosexual
3
Bisexual
4
Ethnicity: ___________________________
Religion: ___________________________
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5
Exclusively Homosexual