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Copyright 1989 by the American Psychological Association, Inc
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Journal of Comparative Psychology
1989. Vol. 103, No. 1.4-22
The Theory of Evolution: Of What Value to Psychology?
Charles B. Crawford
Simon Fraser University Burnaby, British Columbia, Canada
Political considerations and the inadequacies of traditional evolutionary theory for explaining
significant behaviors have discouraged psychologists from studying and using it in the past.
However, the modern synthetic theory of evolution can enrich psychologists' repertoire of
explanatory concepts by providing a critical perspective for analyzing human nature, helping
them to integrate the costs and benefits of learning into their thinking, clarifying the origin and
development of gender differences, and focusing attention on the interaction of humans with
their environment. To illustrate these concepts, examples of each of these functions are presented.
In addition, I describe the concept of adaptation and the methods for its study, with particular
attention to a current study of the evolutionary significance of anorexia nervosa.
Man with all his noble qualities, with sympathy. .. which extends
not only to other men but to the humblest of living creatures,
which has penetrated into the movements and constitution of
the solar system—still bears in his bodily frame the indelible
stamp of his lowly origin. (Darwin, 1871/1898, p. 634)
Few psychologists have trouble with the argument that the
theory of evolution is the explanation for the origin of life
and ought to be taught in high school science classes. But if
the phrase "bodily frame" is interpreted, as Darwin surely
intended, to mean that man's behavior as well as his anatomy
"still bears . . . the indelible stamp of his lowly origin," how
many psychologists will then defend it? Controversy has raged
around Edward O. Wilson, Richard Alexander, and other
evolutionarily oriented researchers because they have been
forcefully arguing that one cannot fully understand human
nature until we explore its biological roots and how they affect
human actions.
Biologists have used ideas from the modern synthesis, which
is an integration of ideas from natural selection and modern
genetics, to help explain a wide variety of animal behaviors.
Numerous anthropologists, sociologists, philosophers, psychologists, and geneticists have become fascinated by the
application of these concepts to the study of human behavior.
Its principles have been used in attempts to understand human helping behavior and altruism (Alexander, 1974, 1979;
Axelrod & Hamilton, 1981; Chagnon & Irons, 1979; Krebs
&Miller, 1985; Trivers, 1971), cooperation (Alexander, 1974,
1979; Trivers, 1971, 1985), incest avoidance (Shepher, 1983;
van den Berghe, 1983), child abuse (Daly & Wilson, 1980),
homicide (Daly & Wilson, 1982), rape (Shields & Shields,
1983; Thornhill & Thornhill, 1983), suicide and self-destructive behavior (de Catanzaro, 1980), war (Alcock, 1984; Shaw
& Wong, 1987) male sexual jealousy (Daly, Wilson, & Weghorst, 1982), grandparental investment (Smith, 1982; Turke &
Betzig, 1985), bequest behavior (Smith, Kish, & Crawford,
The research in this article was supported in part by a Simon
Fraser University Programs of Distinction Grant to Charles B. Crawford.
Correspondence concerning this article should be addressed to
Charles B. Crawford, Department of Psychology, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada.
1987), grief over the death of children (Littlefield & Rushton,
1986), and competition, sometimes involving war, rape, and
resource allocation (Betzig, 1986). Hundreds of articles and
dozens of books on evolution and human behavior have
appeared during the last 10 years.
Books by Wilson (1975) and Alexander (1979) provide
much of the basic theory for applying the modern synthesis
to human behavior. However, psychologists may find Symons's (1979) The Evolution of Human Sexuality the most
stimulating introduction. Trivers's (1985) Social Evolution is
the most up-to-date textbook. For a recent critical review, see
Kitcher(1985).
Psychological Resistance to Evolutionary Theory
Although there are political reasons why psychologists have
resisted incorporating ideas from the theory of evolution into
their thinking, the primary reason they have not used it
effectively is that until recently it did not provide a repertoire
of useful constructs for formulating detailed explanations of
human and animal behavior. Therefore, evolutionary theory
was treated as a convenient framework for discussing adaptiveness rather than as a paradigm for developing testable
hypotheses about behavior. During the last 2 decades, however, the modern synthesis has been enriched by concepts
such as inclusive fitness, kin selection, and the evolution of
life histories, which have made it more applicable to behavior.
Patry (1983) states that "evolutionary theory is of limited
value in formulating hypotheses about developmental processes because the notion that 'ontogeny recapitulates phylogeny' is no longer acceptable" (p. 1026). This quotation
illustrates the fact that many psychologists still are not familiar
with recent developments in evolutionary theory. The relevance of evolutionary thought to developmental psychology,
for example, depends not on the long-discredited notion of
ontogeny recapitulating phylogeny but on the theory of the
evolution of life histories (Gadgil & Bossert, 1970; Hamilton,
1966; Stearns, 1976; Williams, 1957, 1966). This theory states
that the developmental timetable of organisms (the allocation
of resources among survival, growth, and reproduction at
different ages) is a trait shaped by natural selection. Because
it helps psychologists to understand variation in such traits as
EVOLUTION, RELEVANCE, PSYCHOLOGY
growth schedules, age at maturity, and birth spacing and their
interactions with sources of mortality, life history theory may
provide them with insights that will help to integrate the
psychological and biological study of development.
The aim of this article is not to explain in any detail the
many other evolutionary concepts that may useful for psychologists, nor is it to provide a critical review of the application of the modern synthesis to the study of behavior. Its
purpose is to ask in a more general and broad sense whether
psychologists can benefit from an understanding of the modern synthetic theory of evolution. First, however, a brief
review of evolutionary theory is necessary.
The Theory of Evolution
Adaptation and Natural Selection
An adaptation is "an attribute that permits the possessor to
accomplish those immediate objectives that it must achieve
in order to survive and reproduce successfully" (Dunbar,
1982, p. 12). An anatomical structure, a physiological process,
or a behavior pattern that contributes to an individual's ability
to survive and reproduce in competition with other members
of its species is said to be adaptive.
Psychologists, accustomed to thinking of anatomical structures such as the teeth of lions or the hands of humans as
adaptations, are less inclined to regard cub-killing behavior of
male lions (Schaller, 1972) or love in humans (Barash, 1979)
as adaptations that may have evolved through natural selection. A more detailed discussion of adaptations and their
study concludes this article.
Darwin (1859) explains natural selection as follows:
Assumption 1
All species are capable of overproducing offspring.
Assumption 2 The size of populations of individuals tends to
remain relatively stable over time.
Assumption 3 Resources for supporting individuals are limited.
Inference 1 A struggle for existence among individuals ensues.
Assumption 4 Individuals differ in their ability to survive and
reproduce.
Assumption 5 At least some individual variation in this ability
is heritable.
Inference 2 There is differential production or survival of
offspring by genetically different members of
the population, which is by definition natural
selection.
Inference 3: Through many generations evolution of traits
that are more adaptive than others will occur
through natural selection.
In summary, some feature of the environment, the arrival of
a new predator or a change in climate, poses a problem for
organisms. A solution (e.g., longer legs or thicker fur) would
be helpful. The above assumptions and inferences explain
how natural selection provides the solution. Preexisting adaptations, called preadaptations, provide both stepping stones
and limits to the solutions natural selection can provide
(Gould, 1982). Hence, there are constraints on the "perfection" natural selection can provide.
Assumptions 1, 2, and 3 do not require that nature be "red
in tooth and claw." A variety of subtle, and not so subtle,
strategies enable individuals to contribute differentially to the
next generation of individuals. In "Geometry of the Selfish
Herd," for example, Hamilton (1971) argued that animals in
aggregations position themselves to minimize the relative risk
of predation by placing other animals between themselves
and potential predators. Sexually mature Florida scrub jays
may propagate more of their alleles by deferring reproduction
for up to 5 years while they assist their parents in rearing
additional broods (Woolfenden & Fitzpatrick, 1984).
Although genes are involved in all behavioral development,
Assumptions 4 and 5 do not imply genetic preprogramming.
Biologists call traits whose development in individuals is
influenced by environmental factors facultative traits and the
alleles involved in their development facultative alleles. For
example, although male white crowned sparrows cannot learn
the song of another species, they must hear an adult male of
their own species sing while they are nestlings and must
themselves sing while they are juveniles if they are to sing a
full song as adults (Konishi, 1965).
Nor do these two assumptions bar genotype-environment
interactions: The biasing of development in different directions when different genotypes develop in different environments. For example, Cooper and Zubek (1958) reared Tryon's
(1940) "maze bright" and "maze dull" rats in enriched and
impoverished environments. The enriched environment had
no effect on the bright rats, although it improved the performance of the dull rats. The impoverished environment had little
effect on the dull rats but was very detrimental for the bright
rats. Further research, within an evolutionary perspective,
may show that ancestral populations of rats from which the
two strains were developed contained alleles for both bright
and dull behavior and that these alleles influenced fitness in
different environments.
Evolution and Population Biology
This description of the evolutionary process, which does
not involve genetic concepts other than genetic inheritance,
may be called the phenotype view of evolution because it
focuses on the anatomical structures, the physiological processes, and the behavior patterns that were the solutions to
problems encountered in ancestral environments. Explaining
evolution through changes in allele frequencies that are influenced by selection, migration, mutation, drift, and other
population processes is the basis of a more quantitative genotype view.
Although these approaches are complementary, researchers
who use them often differ in their goals and methods. Practitioners of the phenotypic approach favor the comparative
method for explaining the formation of adaptations through
divergent evolution (the divergence of species from a common
ancestor because of differing ecological pressures) and convergent evolution (the convergence of unrelated or distantly
related species because of common ecological pressures).
Because it requires species comparisons, the comparative
method is difficult to use for studying the fine-grained behav-
CHARLES B. CRAWFORD
iors that are of interest to psychologists. Researchers interested
in human behavior must, therefore, use additional methods
for studying the evolutionary significance of behavior. Some
of these methods are based on the assumption that the behavior of ancestral organisms evolved to increase their fitness.
This assumption leads to the development of hypotheses and
models that can be tested through experimentation, field
studies, or computer simulation (Barash, 1982). Biologists,
psychologists, philosophers of science, and anthropologists
who may have learned their evolutionary theory in the context
of comparative anatomy sometimes have trouble relating to
this approach.
Using Evolutionary Theory to Understand Behavior
Human beings have discovered fire and cures for many
diseases. Schools, churches, multinational corporations, and
a host of other civilizing institutions have been devised. It is
thus tempting to conclude that evolution by natural selection
is no longer occurring in human populations, that evolved
behavioral adaptations no longer influence our behavior, and
therefore, that those who study human behavior have no need
for the theory of evolution in their work. Although using the
theory of evolution requires neither that evolution is now
occurring nor that evolved behaviors currently contribute to
fitness, its use does require that behavioral adaptations that
evolved in ancestral populations can influence current behaviors.
Darwin (1898) observed that behavior probably changes
before the physiological structures that mediate it. An attempt
at using tools, for example, may produce selection pressure
for the evolution of more useful hands and a larger brain.
Therefore, the behavior of individuals, which provides the
data on which most current research and theorizing in psychology is based, must be of great importance in understanding evolution in general and the evolution of human nature
in particular. Psychologists have an important role to play in
understanding the evolution of man. It is a role that they have
been reluctant to fill.
How the Application of Evolutionary Theory
Can Benefit Psychologists
Broadens Our Understanding of the Causes
of Behavior
Most psychologists, biologists, and anthropologists are interested in proximate explanations, the immediate factors,
such as internal physiology and environmental stimuli, that
produce a particular response. Evolutionary biologists focus
on ultimate explanations, the conditions of the biological,
social, and physical environment that on an evolutionary
time scale render certain traits adaptive and others nonadaptive (Mayr, 1961). For example, the proximate explanation
of hibernation in ground squirrels is that current climatic
conditions trigger physiological mechanisms that initiate hibernation. The ultimate explanation is that climate, predator
pressure, food supply, and so forth over an evolutionary time
scale make physiological mechanisms mediating hibernation
adaptive. Considering both levels of explanation may deepen
understanding of the development and significance of a behavior.
Psychologists often confuse the use of the terms distal and
ultimate. For psychologists, describing a stimulus as distal
implies that it has a less direct connection with the behavior
in question than some other stimulus that is more directly or
proximally associated with the behavior. In a discussion of
factors affecting a child's IQ for example, Scarr (1985) treated
the mother's IQ as a distal variable and her positive discipline
and control as proximate variables. But ultimate, in an evolutionary sense, implies interpreting an environmental factor
in terms of its influence on evolving adaptations. Therefore,
the mother's IQ is an ultimate cause only if it is interpreted
as an environmental factor influencing adaptations that children are evolving.
Sensation seeking: The "ultimate causes." A more subtle
example is the explanation of the excess in male versus female
mortality, due to disease, trauma and stress, accidents, and
murder, that occurs in humans and many other animals.
Given that excess male mortality is not attributable to the
unguarded X chromosome (Trivers, 1985), the theory of
sensation seeking (Zuckerman, Buchsbaum, & Murphy,
1980) may provide a proximate explanation. Measures of
sensation seeking correlate with those of risk taking in a
number of situations and are also related to a variety of
behavioral variables, such as sexual experience, interest in
new situations, experience with drugs, social dominance, sociability, playfulness, manic-depressive tendencies, and psychopathy. Men score higher than women on sensation seeking, and scores decline with age after age 16. Scores are also
correlated with strength of initial orienting reflex, augmenting
versus reducing of the average evoked brain potential, gonadal
hormones, and the enzyme monoamine oxidase. Many of the
correlates of sensation seeking have moderate to high heritabilities. Zuckerman et al. theorized that the bioamines, particularly the catecholamines, that may regulate the sensitivity
of reward and activity centers in the limbic system play a
central role in this behavior. Finally, depending on the level
of analysis, gonadal hormones, catecholamines, or sensation
seeking can be considered a proximate cause of the sex
differences in mortality. But what are the ultimate causes of
the excess in male versus female mortality?
In most species, including humans, the minimal investment
for a female to reproduce is considerably greater than that
required for a male to reproduce. Therefore, males can benefit
more than females by seeking additional matings. Trivers
(1985) argued that natural selection favors traits that result in
a reduction in survival prior to reproduction if they increase
the reproductive success of those who do survive to mate. For
example, diverting resources from growth of the immune
system to risky behaviors of threat or aggression may increase
the fitness of males if it reduces prereproductive male survival
by one half but more than doubles the reproductive success
of those who survive. The risky behaviors that led Zuckerman
et al. (1980) to postulate a trait of sensation seeking may have
contributed to male reproductive success in ancestral populations.
EVOLUTION, RELEVANCE, PSYCHOLOGY
The ultimate causes of the differential mortality are the
factors that determine the extent to which males can benefit
from seeking additional copulations. These are sex differences
in (a) the cost of constructing a gamete (females usually invest
much more in each gamete), (b) any additional postzygotic
parental effort per offspring (females usually make the larger
investment), (c) effort or resources (dowries and bride prices
in humans and the nuptual gifts provided by some male
insects) exchanged for a mating that limit the ability to
compete for additional mates (males usually invest more;
Thornhill & Alcock, 1983). From an evolutionary perspective
these are the causes of the excess in male versus female
mortality and the sex differences in sensation seeking.
It is possible to construct a model for predicting species
differences in differential mortality and sensation seeking.
Males of species characterized by relatively low postzygotic
male parental investment and relatively high costs of obtaining mates are predicted to exhibit risky anatomy, physiology,
and behavior and to suffer higher mortality than females.
Psychologists might call them sensation seekers. The model
requires neither knowledge of the biochemistry or physiology
of the species nor constructs such as emotions, motivations,
and drives. Instead, it would require information on the
differences between the sexes in investment in gametes, additional postzygotic parental investment, and resources exchanged for a mating.
Value of ultimate explanations. Proximate explanations
tell us how a behavior occurs; ultimate explanations tell us
why it occurs. Of what value are ultimate explanations to
psychologists interested in a single species? A consideration
of ultimate causation may help scientists (a) choose independent variables for the development of within-species models
and theories, (b) gain an understanding of the environmental
factors that may alter a behavior, (c) determine which variables ought to be considered causes and which ought to be
considered effects, and (d) develop explanations with greater
generality. Consider these in the context of the example of
differential mortality and sensation seeking.
If evolutionary theory is to be used in constructing withinspecies theories of variability, researchers must assume that
members of the species have evolved a repertoire of environmentally contingent behaviors for dealing with conditions in
their environment. These behaviors may be either concurrently or developmentally contingent on environmental conditions. Subordinate males who have difficulty attracting
mates because they lack social status and resources may
increase their fitness by increasing the riskiness of their behavior to obtain these attributes. Males with a greater chance
of forming long-term bonds with females because of their
resources and social status may attain greater fitness by pursuing a less risky tactic involving higher parental investment.
The former are expected to make higher sensation-seeking
scores, whereas the latter are expected to make lower scores
on these measures. The risky males are also expected to suffer
greater mortality.
A consideration of the ultimate causes of a behavior may
help unravel the causal sequence of events between a stimulus
and a response. For example, a consideration of the ultimate
causes of sensation seeking suggests that it is a common
element in behaviors that men performed in attempts to
achieve the social status and resources that were necessary for
obtaining mates in an ancestral human population. If this
line of reasoning is followed, then gonadal hormones rather
than the strength of initial orienting reflex, augmenting versus
reducing of the average evolved brain potential, or monoamine oxidase are likely to be an important proximate biological cause of sensation seeking. The ultimate-cause analysis
further suggests that training in social and technical skills is
more likely to be effective than drug therapy in changing
behaviors related to risk taking and sensation seeking. This
suggestion is based on the assumption that humans have a
single genetically organized life history with environmentally
contingent behaviors for implementing it.
Many psychologists prefer explanations that use proximate
rather than distal variables (Scarr, 1985). Sensation seeking,
for example, is apparently a concept derived from inspection
and analysis of observed data. Although concepts developed
in this manner have an intuitive and commonsense appeal
and may function reasonably well as predictors within a
particular context, they lack generality. If psychologists desire
theories of behavior that are not situation and population
specific, they must seek constructs with greater generality. In
contrast to the natural sciences, which can call on theories
such as classical mechanics, the theory of relativity, or the
theory of evolution for guiding the search for explanations,
psychology lacks a framework for guiding its search for more
powerful explanatory theories of behavior. The earlier example illustrates how a consideration of both the proximate and
ultimate causes of a behavior can lead to more general explanations of the behavior and a deeper understanding of it.
Provides a Critical Perspective for Viewing
Psychological Constructs
The following discussions of acting for the good of the
group and intrafamily conflict illustrate how an understanding
of evolutionary theory can help psychologists take a more
critical attitude toward the constructs they use.
Acting for the good of the group.
Though occasionally, in the territorial or rival fights, by some
mishap a horn may penetrate an eye or a tooth an artery, we
have never found that the aim of aggression was the extermination of fellow members of the species [italics added] concerned.
(Lorenz, 1966, p. 47)
Some degree of analytic-cognitive control over the fight-or-flight
response must be necessary in social species where fighting
and fleeing must be regulated for the good of the group [italics
added]. The analysis of such emotion "sending abilities" and
their implications for personality and social functioning in humans is just beginning (Buck, 1979). (Buck, 1985, p. 398)
These authors assumed that individuals evolve to act for the
good of group, as did Wynne-Edwards (1962) when he argued
that selection acts on groups rather than on individuals.
Models of group selection, although attractive to those attempting to understand the paradox of the existence of helping
behavior in many species and the inability of individual fitness
based evolutionary theory to explain its evolution, have usually foundered on the problem of cheating (Williams, 1966).
CHARLES B. CRAWFORD
Consider a group, made up of individuals who sacrifice
some of their reproductive fitness for the good of other group
members, competing with other groups. A group made up of
such individuals may prosper. However, an individual group
member possessing a mutation that programs it to accept, or
enables it to learn to accept, helping behavior without reciprocating that help leaves more offspring than altruistic members of the group. A group or species made up of individuals
acting for the common good is thus inevitably undermined
by mutation from within and by immigration from without.
Groups must appear and disappear at an unrealistically
high rate to make group selection a viable evolutionary process (Maynard Smith, 1976). Although attempts to develop
more sophisticated models of group selection are underway
they do not yet provide a viable alternative to individual and
kin selection (Barash, 1982; Wittenberger, 1981).
The inadequacies of theories of group selection cast doubt
on psychological theories that implicitly assume individuals
behave for the good of the group. For example, many psychologists assume that empathy is a facilitator of altruism and
an inhibitor of aggression (Zahn-Waxier, Cummings, & lannotta, 1986). The vicariously experienced painful consequences of distress in another individual, the argument goes,
cause empathic distress that can be reduced by helping the
distressed individual. The reduction in this stress provides the
motivation for inhibiting aggression, behaving altruistically,
and learning prosocial behaviors. Here the implicit assumption is that empathy is an enabling device for helping others
at a cost to oneself, that is, that empathy is an enabling device
for acting for the good of the group. Those with an evolutionary perspective are not surprised that there is little support for
the prediction derived from this argument that empathy is
positively correlated with prosocial behavior and negatively
correlated with aggressive behavior (Eisenberg & Miller, 1987;
Underwood & Moore, 1982).
An evolutionary perspective, based on the assumption that
individuals evolve to enhance their own fitness or the fitness
of genetic kin, suggests that empathy is an enabling device for
mediating a variety of behaviors ranging from altruism to
avoiding blame and the manipulation of others in one's own
interest or in the interest of others treated as kin by evolved
kin-discrimination mechanisms. As such, empathy may be a
less costly mechanism for attaining one's ends than aggression,
and those high in empathy may be less likely to engage in
aggressive behavior. Moreover, females are expected to be
more empathic than males because physically aggressive behavior is more costly for them. Similarly, those high in
empathy may be feared by those low in empathy and, therefore, receive aggression from them. This perspective also
predicts that empathy leads to prosocial behavior when the
individuals empathized with have characteristics that would
result in their being treated as genetic kin in an ancestral
population. Experiments on empathy, helping behavior, and
aggression ought to be designed to take these possibilities into
consideration.
Within-family conflict. The romantic view of the family
as a loving group of individuals cooperating to bring happiness
to one another has led many to oppose abortion, divorce, and
the division of children between parents after a marital
breakup. A consideration of the evolutionary significance of
the genetic relationships within the family, however, predicts
conflict as well as cooperation within the family group (Trivers, 1974, 1985).
Figure 1 provides the probabilities that parents, grandparents, and children possess an allele (say A or a) that is a copy
of an allele possessed by an ancestor. These probabilities,
known as genetic correlations, are computed by raising .5 (the
probability that A or a is passed on when a gamete is formed)
to the power of the number of arrows connecting two persons.
For example, the genetic correlation between the focal person
(B) and his or her niece or nephew (E) is .52 (or .25), because
they are connected by two arrows.
Trivers (1974) argued that the behavior of individuals,
including parents, children, and grandparents, is shaped by
natural selection to maximize the probability that copies of
alleles they carry are replicated through their behavior. Let us
consider this probability from the point of view of person B
in Figure 1. The probability that B's alleles will be replicated
through his or her offspring (D) is .5, whereas it is only .25
through B's niece or nephew (E). Therefore, natural selection
will favor individuals in B's position who seek an unequal
share of their parent's (A's) resources. Person A, however, is
equally related to all his or her grandchildren and is selected
to resist B's demands.
The intensity of conflict varies as a function of a child's
age. When the child is very young, the parents' fitness is
maximized by investing in him or her, deferring the birth of
additional future offspring. When the child is older and less
dependent for survival on parental attention, parents can
increase their fitness by investing in additional offspring.
Conflict is most intense at intermediate ages, when the parents' fitness is increased more by investing in additional
offspring but the child's fitness is increased more by continued
parental investment.
Sibling rivalry and parental efforts to suppress it as well as
parent-offspring conflict over the amount and timing of
Parent of B and C
Grandparent of D and E
Focal individual
q^
0.5
Offspring of B
0.5
Sibling of B
0.5
Niece or nephew of B
Figure 1. Multiplying along the arrows connecting two individuals
gives the probability that they will share an allele by common descent.
(A grandparent, A, is equally related to all his or her children, B and
C, and to all his or her grandchildren, D and E. However, the focal
individual, B, shares an allele with his or her children, D, with
probability .5 and with his or her nieces and nephews, E, with
probability .25. Thus B is selected to desire an unequal share of A's
resources.)
EVOLUTION, RELEVANCE, PSYCHOLOGY
investment are expected if ontogeny is viewed from an evolutionary perspective. Understanding and dealing with these
problems may be easier once psychologists realize that conflict
as well as helping behavior can be expected in families. For a
discussion of human intrafamily conflict, see Daly and Wilson
(1987).
Enriches Our Repertoire of Explanatory Constructs
The discussion of acting for the good of the group and
intrafamily conflict illustrates how the theory of evolution
can provide a critical perspective on the development of
explanations. It also provides a repertoire of constructs that
may be used in the development of explanations. Two of
these are inclusive fitness and reproductive value.
Inclusive fitness and helping behavior. Hamilton (1964)
provided a putative resolution to the problem created by the
widespread existence of helping behavior in animals and the
inability of individual or group selection to explain it when
he wrote
The social behavior of a species evolves in such a way that in
each distinct behavior evoking situation the individual will seem
to value [italics added] his neighbours' fitness against his own
according to the coefficients of relationship appropriate to that
situation, (p. 19)
Hamilton saw that biological altruism, self-destructive behavior performed for the benefit of others, can evolve, although
it reduces the reproductive fitness of the donor of the help, if
it aids genetic kin, some of whom must inherit the helping
allele from a common ancestor of the donor. More specifically, biological altruism can evolve if the cost to the donor
of a biologically altruistic act is less than the product of its
benefit to the recipient and the genetic correlation (the probability they both have an allele that is a copy of a common
ancestor's).
Now consider, for example, the interactions between a
donor, its full sibling, and its half sibling. The donor performs
an altruistic act, reducing its own reproductive success from
four to three offspring but increasing the reproductive success
of the full sibling from three to five and that of the half sibling
from two to six offspring. The cost to the donor is one
offspring, but the benefit to the donor is 0.5(2) + 0.25(4) = 2
offspring. The donor's actions result in the equivalent of five
offspring rather than the four direct descendants that would
have been produced without the help directed to genetic kin.
Alleles enabling individuals to perform nepotistic acts (helpful
acts directed to genetic kin) can spread by natural selection.
The expanded notion of fitness, known as inclusive fitness,
can be defined as the sum of (a) an individual's direct fitness,
its personal reproductive success, stripped of the effects of
actions of genetic relatives and (b) the individual's indirect
fitness, its influence on the reproductive success of genetic
relatives, with the relatives weighted by the degree of genetic
relation to the individual. Inclusive fitness is one of the most
powerful concepts that evolutionary theory has to offer psychology for understanding helping and conflict behavior. It
has helped explain phenomena as diverse as sterile castes in
social instincts (Hamilton, 1964), coalition formation in pri-
mates (Baker & Estep, 1985), and human bequest behavior
(Smith et al., 1987) by focusing attention on the behavioral
consequences of genetic kinship and mechanisms for discriminating kin from nonkin.
Kin recognition. If social organisms have evolved the capacity to put their helping behavior where their alleles are,
they must possess some proximate mechanisms for kin recognition, and these ought to be of interest to psychologists
because they provide an important point of interface between
psychology and evolutionary theory. Possible mechanisms for
kin recognition include spatial distribution (treating individuals in a prescribed geographic area as kin), association (treating frequent associates as kin), phenotype matching (treating
individuals similar to oneself or similar to those that one was
raised with as kin), and recognition alleles (innately treating
individuals with a particular genetic marker as kin). They
have received attention from biologists (Holmes, 1986; Waldman, 1986) and psychologists (Rushton, Russell, & Wells,
1984, 1985).
If small kinship groups were an important feature of human
social structure during our evolution, then one or more of
these kin discrimination mechanisms may have evolved and
may still be influencing our behavior. Understanding the
mechanisms involved may be critical if psychologists wish to
change human behavior through social learning. For example,
if spatial distribution or association evolved as mechanisms
of human kin discrimination, then encouraging individuals
of different ethnic groups to live together in the same geographic area may contribute to ethnic harmony. However,
other kin discrimination mechanisms may complicate the
process of achieving ethnic harmony. If treating persons similar to those one was raised with is a human kin discrimination
mechanism, then ethnically integrating schools starting with
preschools rather than high schools is more likely to lead to
ethnic harmony. If treating persons similar to oneself as kin
was an aspect of kin discrimination among our ancestors,
then it may be necessary to encourage different ethnic groups
to adopt similar accents, manners, customs, clothing, and so
forth to increase ethnic harmony. Finally, if genetically produced signs evolved to discriminate among kin and nonkin
among our ancestors, then producing ethnic harmony may,
indeed, be difficult because social learning is not involved.
Both theoretical and empirical work on the role of social
learning in kin discrimination are needed if the concept of
inclusive fitness is to be applied to human behavior. This
work is proceeding. Hepper (1986) provides an up-to-date
review of the biological literature on kin discrimination, and
Porter (1987) reviews the little that has been done on human
kin discrimination. Little work on the relation between inclusive fitness, kin recognition, and concepts from social psychology has appeared. It is a fruitful area for interdisciplinary
research.
Reproductive value and behavior. Fisher (1930) developed
the concept of reproductive value, defined as the relative
number of female offspring remaining to be born to a female
of a given age, to quantify an individual's value in an evolutionary context. For example, the number of female offspring
still to be born to 1,000 one-day-old females is less than
number to be born to 1,000 one-year-old females because
10
CHARLES B. CRAWFORD
some of the one-day-olds will die before reaching one year of
age. Hence, the one-year-old females have greater reproductive value than the one-day-old females. Similarly, a group of
50-year-olds have less reproductive value than a group of 25year-olds because members of the latter group can expect to
produce more offspring during the remainder of their lives.
Thus, reproductive value rises during the early years of life,
because of infant and juvenile mortality, peaks around the
age of sexual maturity, and then begins a decline. Although
usually computed for females, it can be computed for males
if age-specific birth and death rates for males and the stable
age structure of the male population are known.
A variety of demographic variables influences reproductive
value. For example, in a growing population a young female's
reproductive value is devalued relative to an older female's
value because the younger female's progeny make up a smaller
proportion of the next generation then do the older female's.
Similarity, if the size of the population is declining, the
reproductive value of the younger female is relatively greater
because her offspring will make up a greater proportion of
the next generation. In humans the male and female reproductive value curves differ somewhat because the age-specific
birth and death rates differ somewhat for men and women.
Women, for example, both begin and cease producing offspring at an earlier age than men. Men die at a faster rate at
all ages than do women.
Figure 2 shows the female reproductive value curves for
Keyfitz and Flieger's (1971) data for several industrialized
and developing nations. Note the difference between the
curves for Japanese and Mexican women for the year 1966.
From an evolutionary perspective the same chronological age
has a different meaning for each group. Even the curves for
Japan in 1964 and 1966 differ. Reproductive value curves
apparently reflect important ecological features of the populations they represent.
Because individuals' reproductive value is a measure of
their ability to propagate their alleles and because an essential
assumption of the theory of evolution is that individuals
evolve to behave in a way that maximizes the number of their
alleles propagated, it follows that adaptations for assessing
and responding to the reproductive value of oneself and others
may have evolved. It is not adaptive, for example, for humans
to be sexually attracted to members of the opposite sex who
are of low reproductive value.
It is well known that humans respond to individual differences in physical and psychological characteristics, such as
clear skin, bright eyes, vitality, and social skill. If such characteristics are really being used as cues to reproductive value,
then our emotional and cognitive reactions to them may be
the product of natural selection, and reproductive value may
be at least as important as age in understanding human
behavior. However, most psychologists appear to be unaware
of its power.
Figure 3 shows the intensity of grief that undergraduates
expect parents to experience at the death of daughters of
different ages compared with reproductive values for the
Province of British Columbia and for IKung Bushwomen
plotted against age. The grief values were obtained by using
Thurstone's (1927) law of comparative judgment. The reproductive values were computed from Canadian census data
and from Howell's (1979) demographic data on the Dobe
IKung. The linear component of the correlation of pooled
female grief scores with reproductive values for British Columbia females is 0.65. The correlation of the same grief values
Mexico 1966
Japan 1964
Japan 1966
Reproductive
value
10
15
20
35
30
35
40
45
50
55
Age (years)
Figure 2. Female reproductive value functions for several contemporary societies. (The number of
female offspring that females of various ages can be expected to produce during the remainder of their
lives is plotted against maternal age. Data from Keyfitz & Flieger, 1971.)
EVOLUTION, RELEVANCE, PSYCHOLOGY
11
Grief for daughter
Female reproductive value,
British Columbia
Scaled grief 0 7 _|
and
reproductive °-e ~
values
Female reproductive value,
Dobe IKung
10
15
20
25
30
35
40
45
50
Age (years)
Figure 3. Intensity of grief (determined by Thurstone's, 1927, method of comparative judgment) for
the death of children varying in age from 1 day to 50 years. (The reproductive values were computed
from British Columbia, Canada, census data from 1981 and from data collected by Howell, 1979, for
Dobe !Kung. The raters were 174 undergraduate university students.)
with reproductive values for IKung Bushwomen is 0.95. Because the curves differ only for young children, it is tempting
to speculate that the intensity of grief experienced at the death
of a child reflects the reproductive schedule of our huntergatherer ancestors.
Ideally, both reproductive value and inclusive fitness ought
to be used when considering interactions between individuals.
If they are to be used jointly, it is necessary to distinguish
between direct and indirect reproductive value, just as it was
necessary to distinguish between direct and indirect fitness
when considering interactions between kin. Direct reproductive value is the number of offspring still to be born to a focal
individual of a given age. Indirect reproductive value is the
number of offspring still to be born to an individual of another
age with a genetic correlation to the focal individual. A
postmenopausal woman, for example, has zero direct reproductive value but may have considerable indirect reproductive
value if she has interactions with an extended kin group.
Now consider to which of three cousins, ages 1 week, 15
years, and 45 years, a childless donor ought to bequeath an
estate to maximize the donor's fitness. Because the potential
recipients are equally related to the donor, a consideration of
kinship does not provide useful information. As the donor is
childless, a consideration of the donor's direct reproductive
value provides no useful information.
If the persons were citizens of a developing nation with
high infant mortality, a consideration of the indirect reproductive value of the donor (the direct reproductive values of
the cousins; see Figure 2) suggests the estate ought to go to
the 15-year-old. However, if the individuals were members of
some highly developed country with low infant mortality, the
same consideration suggests the estate should go to the 1week-old.
Suppose the estate was to be left to either a 1-week-old
grandchild, whose genetic correlation with the donor is .25,
or a 15-year-old cousin, whose genetic correlation with the
donor is .125. Now both the indirect reproductive value of
the donor and the degree of kinship of the donor and beneficiaries must be considered. On the basis of joint considerations of both reproductive value and kinship, the person in
the developing country may be expected to favor the 15-yearold cousin, whereas the one from the developed country may
be expected to favor the 1-week-old grandchild. Finally, a
relatively young childless donor with high direct reproductive
value might respond differently from an elderly childless
individual with zero direct reproductive value. Note that all
these computations assume humans have evolved psychological mechanisms for assessing kinship and reproductive value.
In Kreb's (1970) review of the literature on altruism, he
found benefactor effects for sex, age, ordinal position, social
class, and nationality and recipient effects for friendship status, in-group affiliation, and social class. These results are not
surprising to anyone with a knowledge of evolutionary theory.
The use of inclusive fitness, mechanisms of kin discrimination, and reproductive value may be of considerable use to
psychologists in the development of more general theories of
altruism and helping and conflict behaviors. Moreover, although it may place some limits on the way cognitive and
social learning theories are formulated and used, it does not
preclude them as proximate explanations.
Focuses Attention on the Costs and Benefits of
Learning
Although the benefits of the flexibility and adaptability
made possible by learning are well known, its costs are less
12
CHARLES B. CRAWFORD
often appreciated. Learning requires delicate neurological
structures, which are energetically costly to develop and maintain. An animal may learn fitness-reducing information, as
when a Norway rat or a human learns to avoid an edible food
because on first contact it was tainted or associated with a
tainted food. Humans and other animals may exploit learning
by providing false information for others to learn (Alexander,
1979). Given the costs and benefits associated with learning,
it is unlikely that any species, including Homo sapiens, has
completely general learning abilities.
From the premise that a complex environment offers many
opportunities for learning fitness-reducing responses, Lumsden and Wilson (1981, 1983) have argued that a species
without genetic constraints on the learning of culture must
evolve toward having such constraints. Moreover, the speed
of the evolution is a function of the complexity of the environment. Thus natural selection will equip the mind with
specific rules and principles for learning about the world in
an advantageous way. Lumsden and Wilson have argued that
brother-sister incest avoidance, the learning of color vocabularies, infant preferences from object shapes and arrangements, the development of facial recognition, fear of strangers,
mother-infant bonding, and the acquisition of the phobias
are the result of problems encountered by our Pleistocene
ancestors.
Every graduate student in psychology knows and admires
Garcia's (Garcia, Hankins, & Rusiniak, 1974) work on the
learning of biologically relevant associations, yet it has had
little impact on theory and practice in psychology. Although
the concept of genotype-environment interaction is taught to
introductory psychology students, they are also taught, sometimes in the same course, that if men and women are to
achieve equally as adults they must be treated in the same
way as children. An evolutionary perspective can help psychologists integrate an understanding of the costs and benefits
of learning and genotype-environment interactions into their
thinking.
Cosmides and Tooby (1987) recently took up the challenge
of developing a framework for evolutionary psychology, a
discipline concerned with exploring the naturally selected
design features of the mechanisms that control behavior.
Arguing that cognitive processes are the most essential proximate mechanisms, they define Darwinian algorithms (Cosmides, 1985) as innate specialized learning mechanisms that
organize experience into adaptively meaningful schema or
frames. When activated by appropriate environmental and
proprioceptive information, these mechanisms focus attention, organize perception and memory, and call up specialized
knowledge required for domain-appropriate inferences, judgments, and choices. They suggest that aggressive threat, mate
choice, sexual behavior, pair bonding, parent-offspring conflict, kin recognition, friendship, disease avoidance, resource
distribution, and social contracts are only a few of the domains
of human activity regulated through Darwinian algorithms
that evolved in response to ancestral problems.
Cosmides and Tooby (1987) argued that ancestral environments selected not for behavior but for mechanisms producing behavior, just as these environments selected mechanisms
for detoxifying the blood or extracting oxygen from the atmosphere rather than the actions of these organs. These
mechanisms were selected because they contributed to average
fitness in ancestral populations. In the study of the naturally
selected design of these mechanisms, we can use the theory
of evolution to help understand the human mind. However,
they go on to argue that it is at the cognitive rather than the
physiological level where psychological explanations must be
sought. Adaptive behavior is predicated on adaptive thought,
and adaptive thought can only be understood at the level of
information processing. Although the information processing
mechanisms must be instantiated in neural hardware, a
knowledge of that hardware tells us little about how ancestral
animals solved their problems and how the evolved information processing mechanisms function today.
To test this theory Cosmides (1985) studied content effects
on the Wason selection test, an experimental paradigm for
studying whether people reason in terms of the contentindependent principles of formal logic (Wason, 1968). If the
content of a logical problem affects reasoning, then a content
effect is said to have occurred. She argued that in the context
of social contracts, reasoning ability may reflect the action of
Darwinian algorithms that evolved in our ancestors for dealing with associates who cheated on social contracts by accepting the benefits of the contract but not paying its costs.
She carried out a number of experiments that compared
predictions from Darwinian algorithms with those of availability theory, the theory that familiarity with the content of
the statements used in testing reasoning affects errors in
reasoning. She varied content (familiar vs. unfamiliar) and
instructions (prescriptive vs. descriptive). The results strongly
supported her evolutionary perspective in that the errors in
logic were predicted much more accurately on the assumption
of Darwinian algorithms than on the assumption of availability.
Cosmides and Tooby's (1987) focus on the invariance of
cognitive mechanisms rather than on the invariance of behavior has several benefits for those wishing to use an evolutionary perspective in their work. It focuses attention on the
ancestral conditions under which mechanisms mediating behavior evolved and makes it clear that behavior in ancestral
populations had only to be adaptive, on average, for the
evolution of mediating mechanisms to occur. Hence, those
using the evolutionary perspective need not assume that every
action must contribute to fitness or that all behaviors are
currently adaptive. This realization makes it possible to use
the theory of evolution for studying a wide variety of normal
and abnormal behaviors.
The focus on proximate cognitive mechanisms alleviates
the necessity of assuming that behavior is under stimulus
control and thereby brings the evolutionary approach to
behavior into the mainstream of current psychological thinking. Mental processes, if they are clearly defined as Darwinian
algorithms, can become objects for evolutionary analysis.
Just as hands are not constraints on eating, Darwinian
algorithms are not constraints on learning. Both evolved in
ancestral populations for enabling individuals to deal efficiently with conditions in their environment. Hence, from an
evolutionary perspective it is incorrect to speak of constraints
on learning. Moreover, Cosmides and Tooby's (1987) arguments lead to the expectation that the mind comprises numerous, specific, complex mechanisms rather than simple,
EVOLUTION, RELEVANCE, PSYCHOLOGY
general processes of association and symbol manipulation—
a view shared by Symons (1987) and Fodor (1983).
Psychologists have been leery of sociobiologists and evolutionary ecologists because of their concentration on ultimate
or "why" questions to the exclusion of proximate or "how"
questions. Cosmides and Tooby's (1987) work is ground
breaking because it shows us how to use the evolutionary
approach in the study of proximate questions without becoming involved in the details of physiology and neurochemistry.
In the past psychologists using the theory of evolution in their
work have tended to concentrate their attention on emotion
(e.g., Plutchik, 1980). Cosmides and Tooby (1987) expanded
the use of evolutionary theory in psychology by bringing it to
bear on the fine structure of the mind.
Helps Us Understand Sex
Differences
There are at least two sources of selection pressure that
interact and complement each other in producing evolved
sexual dimorphism. The production and rearing of offspring
may have resulted in some specialization of roles for ancestral
males and females, with resulting selection for sexual dimorphism in physical body size, behavior, and brain development (Kimura, 1987; Tooby & DeVore, 1987). The other
evolutionary explanation for sexual dimorphism is Darwin's
theory of sexual selection.
Sexual selection and sexual dimorphism. Sexual selection
refers to differences in the ability of individuals with different
genotypes to acquire matings. If, for example, tall and short
males have similar abilities to survive, but tall males obtain
more matings, then sexual selection is said to be acting on the
males. Darwin (1871/1898) argued that it involves two processes that are usually referred to as intrasexual selection and
epigamic selection.
In intrasexual selection the competition between males for
access to females produces selection pressure for increased
physical size, organs of threat, and aggressiveness in males
and differences between males and females on these traits.
Male elephant seals, for example, compete vigorously among
themselves for access to females. They are about 60% larger
than females and about four times as heavy. A few males may
have as many as 100 females in their harems; most never get
near a female during their entire lives (LeBoeuf, 1974). Epigamic selection is the result of female choice generating
competition between males resulting in the elaboration of
traits that females desire. The tail of the peacock provides the
classic example.
But why do males compete for females, and do females
ever compete for males? In the earlier discussion of sensation
seeking, I wrote that males can usually benefit more than
females from additional copulations because a male's minimal
investment required to reproduce is usually less than a female's. Therefore, females are a limiting resource for males,
and males can benefit by competing among themselves for
access to that resource. Males are usually, but not always, the
sexually selected sex.
Exceptions that prove the rule are provided by organisms
with sex role reversal such as seahorses, certain frogs, and
several species of marsh and shorebirds (jacanas and phalaropes). In these species, although males make a small invest-
13
ment in sperm, their relative minimal parental investment is
large because they take sole responsibility for parental care
(Barash, 1982). As a result, the females have evolved to be
larger or more colorful than the males and to compete among
themselves for access to males.
Sexual dimorphism in anatomy, physiology, behavior, and
especially variance in reproductive success are evidence that
that sexual selection has acted on a species. In humans sexual
dimorphism exists in size (men are larger), in physical aggressiveness (men are more physically aggressive), life span
(women live longer), mortality (men have greater mortality
rates at all ages), date of maturity (women mature earlier),
and minimal parental investment (women make the greater
minimal parental investment). Throughout history most individual men have mated monogamously. However, in a
sample of 849 societies, 83% are either usually or occasionally
polygynous, 16% are monogamous, and 0.5% are polyandrous (Daly & Wilson, 1983). Even in monogamous societies
such as the Hutterites, variance of men's reproductive success
exceeds that of women (Crawford, 1984). This evidence indicates that humans are a moderately sexually selected species.
Psychologists have studied sex differences in behavior for a
century, but they have shown little interest in their evolutionary origin. For example, since January 1986, the Psychological
Bulletin has published five major articles on gender differences
in behavior: human motor activity level (Eaton & Enns,
1986), masculinity, femininity, androgyny, and cognitive performance (Signorella & Jamison, 1986), gender and helping
behavior (Eagly & Crowley, 1986), gender and aggressive
behavior (Eagly & Steffen, 1986), and sex differences in
unipolar depression (Nolen-Hoeksema, 1987). Although the
authors occasionally referred to proximate biological factors
as possible causes of gender differences, not one mentioned
their possible evolutionary significance!
Since the time of Darwin, the theory of sexual selection
and observed sexual dimorphisms in anatomy and behavior
have led many to conclude that males are at the cutting edge
of evolutionary change and that females are passive and
noncompetitive. However, the nature and extent of femalefemale competition is now becoming a topic of major importance to evolutionary ecologists, primatologists, and anthropologists (Hrdy, 1981;Wasser, 1983).
Equality of the Sexes. Although evolutionary theory tells
us why many species exhibit sexual dimorphism in physique
and behavior, it also tells us that males and females are equal
in a biological sense. Fisher (1930) proved that if individuals
in a species have one parent of each sex, it then follows that
the amount of parental investment put into males and females
must be the same at the time offspring become independent
of their parents. (See example in Table 1.)
Suppose that it costs two units of maternal time and energy
to rear a daughter to independence and one unit to rear a son
to the same point in development and that a mother has 100
units of parental care available for rearing offspring. If she
invests in an equal number of sons and daughters, her 100
units of parental care produce 33.3 (33.3 Daughters X 2 Care
Units) daughters and 33.3 (33.3 Sons x 1 Care Unit) sons for
a total 66.6 offspring. As each grandchild must have one
parent of each sex, the 33.3 sons and 33.3 daughters give her,
on average, the same number of grandchildren. Our hypo-
14
CHARLES B. CRAWFORD
Table 1
Number of Offspring Produced by 100 Units of Parental
Investment for 1:1 and 1:2 Sex Ratios
Investment
strategy
Produce equal sex
ratio
Males
Females
Total
Produce sex ratio inverse to ratio
of unit costs
Males
Unit
cost
Number
produced
Expenditure
2
1
33.33
33.33
66.66
33.33
66.66
100
Females
25
50
50
50
Total
75
100
thetical female may expect 66.6 X 66.6 or 4,435.56 grandchildren.
Now consider another female in the same population who
puts two units of investment into each of 25 daughters and
one unit into each of 50 sons. The same 100 units of parental
care now bring 25 daughters and 50 sons to independence.
Each of her offspring, on average, will give her the same
number of grandchildren. Such a "wise" female can expect
75 x 75 or 5,625 grandchildren.
Thus natural selection produces an equilibrium sex ratio
inversely proportional to the investment in males and females:
the Cost of Males x the Proportion of Males = the Cost of
Females x the Proportion of Females. If the sex ratio of a
species is 50:50 at the time of offspring independence, then
the cost of bringing males and females to independence must
be the same.
If at conception the amount of investment put into males
equals the amount put into females, if the sex ratio of the
species is 1:1, and if offspring competence is a function of
parental investment, then at the time of independence from
the parents, the average competence of males and females
must be the same. In this context competence refers to any
trait enhancing inclusive fitness, such as the ability to acquire
a mate, to breed successfully, to rear offspring, to detect
predators, to acquire food, to learn a skill such as hunting,
gathering, or growing vegetables, to recognize kin, to detect
liars, to cooperate with others, and so forth.
The fact that males and females are biologically equal in
the sense that they receive equal investment does not imply
that they are biologically identical, that patterns of investment
in them are identical, or that similar patterns of investment
produce similar competencies. Males, for example, may require more food, whereas females may require more protection from predators. It may be easier to teach males to be
physically aggressive and females to be good caregivers. The
theory of sexual selection discussed earlier suggests some ways
in which evolved differences in male and female competencies
may come about.
Finally, suppose some external agency provided additional
investment to only one sex, say, for example, males. If this
investment occurred before offspring independence, males
would be less expensive, and natural selection would favor
the production of more of them. Similarly, if the cost of
females were lowered, more would eventually be produced.
Can mechanisms evolve for facultatively adjusting investment
in male and female offspring, and hence their competence, as
a function of the environmental fluctuations that temporarily
alter the costs of producing males and females? This question
is considered in the next section.
Sex allocation. Suppose that an organism desired many
grandchildren and that it did not care whether they were
males or females, should it then invest more in its male or its
female offspring? This is the question posed by sex allocation
theory, which when applied to mammals, describes how an
individual can maximize its fitness by partitioning its investment between its male and female offspring (Charnov, 1982).
Fisher's (1930) theory that natural selection adjusts the sex
ratio to produce equal investment in males and females
requires a key assumption that parents do not have information on the likely reproductive success of their offspring.
Suppose the females of a hypothetical species leave approximately the same number of offspring (low variance in female
reproductive success), but that some males leave many offspring because their physical condition or social status enables
them to compete successfully for many females, whereas other
males leave few offspring because they lack these attributes
(high variance in male reproductive success). Now, further
suppose that offspring inherit some of their mother's health
or social status. Given these suppositions, a mother in poor
health or of low social status can improve her fitness by
investing in daughters at the expense of sons, because her
weak or subordinate sons are likely to give her fewer grandchildren than her weak or subordinate daughters. Similarly,
a healthy or dominant mother can increase her fitness by
investing in sons at the expense of daughters, because her
healthy or dominant sons are likely to give her more grandchildren than her strong or dominant daughters.
Therefore, an adaptation that uses information about one's
own physical condition or social status to allocate investment
in male and female progeny is expected in species in which
the variance of reproductive success is greater for one sex than
for the other (Trivers & Willard, 1973). Because variance in
male reproductive success exceeds that of females in many
animal species, including humans, adaptations for sex allocation may exist.
The sex ratio of progeny is usually used as the measure of
allocated parental investment. Trivers and Willard (1973)
cited evidence supporting their theory from a variety of species, including pigs, sheep, mink, seals, deer, and humans. A
striking piece of evidence supporting Trivers and Willard is
McClure's (1981) finding that when lactating wood rats are
put on a food-restricted diet, they selectively eliminate the
males from their litters. Females can improve their fitness by
beginning allocation of resources to male and female offspring
as early in development as possible. Recently Wright, Crawford, and Anderson (1988) reported evidence that suggested
that dietary stress can induce mice to alter the sex ratio of
their progeny during gestation.
Evidence that Trivers and Willard's (1973) theory applies
to humans is beginning to accumulate. Tentative evidence
EVOLUTION, RELEVANCE, PSYCHOLOGY
suggests that parental ability to invest in offspring may influence the sex of human offspring (Clutton-Brock & Albon,
1982). Dickemann (1979), Voland (1984, 1988), and Boone
(1986) have found evidence that supports Alexander's (1974)
suggestion that in highly stratified societies male-biased sex
ratios are produced among high-status parents by female
infanticide, abuse, and neglect. Dickemann, van den Berghe
and Mesher (1980), Smith et al. (1987), and Boone (1986)
have provided evidence that suggests individuals consider
their own social status when bequeathing wealth to their male
and female kin. Betzig (1986) provides strong support for the
theory of Trivers and Willard from a large cross-cultural
sample.
A recent study by Betzig and Turke (1986) may be of
particular interest to psychologists, who are less interested in
the sex ratio than in patterns of differential investment that
may be involved in the development of sex differences in
behavior. They studied parent-child associations on a small
Pacific atoll south of Guam and found that although the sex
ratio did not differ among high- and low-status parents, higher
status parents spent more time with sons, and low-status
parents spent more time with daughters. These results, which
are in agreement with the findings of the studies of bequest
behavior already mentioned, ought to be of considerable
interest to anyone concerned with the development of sex
differences in personality, abilities, and gender roles.
Focuses Attention on Ecology
Evolutionary theories of social behavior have considered behavior to be like other biophysical structures [italics added] that have
evolved in natural history. One upshot of this reification—
viewing behavior as structure—is that behavioral propensities
are assumed to be conserved, unchanged, across generations.
Since the biological substrates for, say, aggression and altruism,
have evolved over hundreds or thousands of generations, it has
been assumed that they are inevitable [italics added] in the
ontogeny of each person. (Zahn-Waxier et al., 1986, p. 82)
This quotation, from a recent book on the biological and
social origins of altruism and aggression, is one of many
illustrating that psychologists, as well as other scientists and
scholars, often confuse the ideas and methods of classic ethology with those of sociobiology, behavioral biology, and
evolutionary ecology.
The study carried out by the classical ethologists of behaviors and biophysical structures that are "conserved, unchanged, across generations" is fundamental for our understanding of life through natural selection. However, it is
focused on the evolution of between-species differences and
similarities in behavior at the expense of within-species behavioral variation.
The fact that a trait has evolutionary significance does not
imply that it cannot be affected by environmental conditions,
that it must have high heritability, or that it must appear the
same in all individuals. Evolutionary theory focuses attention
on the interaction of organisms with their physical environment and the actions and products of other organisms living
in it. For example, when animals engage in what appears to
be destructive behavior, such as infanticide, they may do it
because it contributes to their lifetime reproductive success,
15
given the ecological circumstances under which they are living
(Hausfater & Hrdy, 1984).
Let us briefly examine some of the concepts currently being
used by ecologists to help explain how biological adaptations
enable organisms to deal with varying conditions in their
physical and social environment. A life history strategy is a
genetically organized program for allocating resources to survival, growth and reproduction throughout an organism's life.
Tactics are decision rules and methods for achieving the life
history (Gross, 1982). Within a population strategies are in
competition with each other; tactics are the way they compete.
The members of a population may have one or more strategies
each with its own tactics for competing with other strategies.
Tactics may be either concurrently or developmentally contingent on environmental circumstances.
Some examples may help clarify these ideas. Male scorpion
flies have three mating tactics: They may obtain a dead insect,
present it to a female, and copulate with her as she eats it;
they may generate a salivary mass, present it to the female
and copulate with her as she eats this nuptial gift; or if they
cannot obtain a dead insect or generate the salivary mass,
they may attempt a forced copulation. Thornhill (1980) has
shown that all three tactics are available to adult males and
that success in male-male competition determines the tactic
used. Even dominant, successful males become vigorous
forced copulators if they cannot successfully compete for a
nuptial gift. The mating behavior of male scorpion flies is
thus an example of a single mating strategy with concurrently
contingent tactics because the tactics used depend on current
conditions in the environment.
Figure 4 illustrates this adaptation for adjusting male behavior to the level of male-male competition. It is drawn on
the assumption that all males have alleles for all behaviors
and the behavior exhibited is contingent on conditions in the
environment. A zero heritability is expected for the behavioral
differences because they are completely determined by environmental differences. However, a nonzero heritability may
be found in a standard behavior genetic study if, for example,
body size is heritable and is correlated with the tactics used.
Tactics may also be contingent on the environment during
development. Male gorillas apparently succeed to breeding
rights within their natal troop or seek matings elsewhere
Environment
Mating tactic
(Male-male competition)
Dead insect + courtship
Low
Medium
,, High
Proteinaceous mass + courtship
h = 0.0
Attempted forced copulation
Figure 4. Mating tactics in scorpion flies (Panorpa sp.). (The level
of male-male competition determines the reproductive tactic used.
Heritability of the tactics is zero because all males have alleles for all
tactics.)
CHARLES B. CRAWFORD
16
depending on their relationship with the dominant male
during infancy and adolescence (Harcourt & Stewart, 1981).
Males with a close social relationship with the dominant male
remain in their natal group, whereas males without such a
relationship leave to compete for mates elsewhere. The tactics
of gorillas are developmentally contingent because they depend on environmental conditions present during development.
It is not known whether gorillas using different tactics differ
genetically. However, it seems likely that the tactics are enabling devices for a single genetically organized reproductive
strategy because the heritability of traits closely related to
reproduction is usually low (Falconer, 1960; Mousseau &
Roff, 1987).
Male bluegill sunfish have two genetically different life
histories, parental and cuckolder, each with its set of tactics
for competing with the other life history (Gross, 1982). The
parental males grow slowly, mature later than cuckolders,
build nests, court females, and provide paternal care once the
eggs are fertilized. The smaller cuckolders grow rapidly and
never provide parental care. When they are small, they attempt to sneak copulations by darting in and releasing sperm
just before the parental male releases his sperm. When they
are larger, sneaking copulations becomes difficult, so they
mimic females and attempt to release sperm before the parental male can release his sperm. Figure 5 diagrams the genetic
and environmental effects for the bluegill sunfish mating
system. Note the difference between the parental and cuckolder strategies has a nonzero heritability, but the difference
between the sneaker and mimic tactics has zero heritability
because it is the size of the individual that determines the
tactic used, not its alleles.
Draper and Harpending (1982) have applied this type of
thinking to the study of the effects of father absence on human
development. They argued that the type of family environment experienced by a child before the age of 5 affects the
reproductive tactics pursued in later life. The child with low
father involvement is being prepared for life in a society where
men frequently compete for access to a number of women
and do not form enduring bonds or provide much investment
in their offspring. The child with high father involvement, on
the other hand, is developing attributes enabling it to maxi-
Genetic strategy
Behavioral tactics
Nest + courtship f care
Parental
h 2 > 0.0
Cuckolder ^S.
Sneaker
»
Female mimic
»
h = 0.0
Figure 5. Mating strategies and tactics in bluegill sunfish (Lepomis
macrochirus). (The difference between life history strategies is heritable. The difference between tactics used to implement the life history
is determined by environmental conditions.)
mize its reproductive success in a society where men form
long-lasting relationships with one woman and provide a high
level of investment in their offspring. From this perspective
the effect of father absence is an example of a developmentally
contingent tactic.
Recall that Darwinian algorithms (Cosmides, 1985) are
innate, specialized learning mechanisms that organize experience into adaptively meaningful schema or frames. When
activated by appropriate environmental and proprioceptive
information, they focus attention, organize perception and
memory, and call up specialized knowledge required for domain-appropriate inferences, judgments, and choices. In the
context of life history theory, they are mechanisms that mediate environmentally contingent tactics by processing information. The three mating tactics of male scorpion flies, for
example, can be seen as the result of a Darwinian algorithm
for processing information about the male's relation to other
males in its environment and producing an appropriate action
in response to that information. Similarity, the higher verbal
abilities of boys reared in father-absent homes (Draper &
Harpending, 1982) can be understood as the reflection of
algorithms that develop in the context of children's experience
and contribute to the success of a particular adult reproductive
tactic.
Heritability and genetic involvement in development. It is
sometimes assumed that if identical twins who are reared in
different environments develop different abilities or personalities, then genes are not important in the development of
the traits in question. A consideration of the mating behavior
of identical triplet scorpion flies reared in three environments
differing markedly in the level of male-male competition
illustrates why this belief is incorrect. Males in the low malemale competition environment exhibit the dead-insect tactic.
Males in the moderate competition environment exhibit the
proteinaceous-mass tactic. Males in the high male-male competition environment may forgo courtship and attempt to
force a copulation. The forced copulation tactic is not seen in
an environment with low or moderate male-male competition. Although the heritability of the behavioral differences is
zero, genes control the development of the mechanism for
adjusting the behavior to environmental conditions. From an
evolutionary perspective the primary issue is not the heritability of a trait but its correlation with inclusive fitness in the
environment in which it occurs or, if the environment has
changed, the environment in which it evolved.
An ecological perspective suggests that psychologists who
wish to use the theory of evolution in their work ought to
initially focus their attention on the development of environmentally contingent tactics. Progress may be most rapid if
they concentrate on behaviors (a) that have low heritability,
(b) that are closely related to reproductive function, and (c)
for which sensitivity to environmental conditions may have
contributed to inclusive fitness in an ancestral population.
The operation of contingent environmental tactics and the
algorithms mediating them can be seen most clearly when
there is no genetic variation, and hence zero heritability, for
the behavior in question. Behaviors closely related to reproductive function are likely to be energetically expensive for
the organism; therefore adaptations making them contingent
EVOLUTION, RELEVANCE, PSYCHOLOGY
on environmental conditions are likely to have evolved. The
final requirement is necessary because the environment of
some species, such as humans, has changed considerably in
recent evolutionary history, so it is necessary to consider the
ancestral environment in which the behavior in question
evolved when studying contingent tactics.
Evolutionary significance of behavior. Psychologists may
gain valuable insights by studying the evolutionary significance of behaviors, the conditions in the ancestral environment of a species that may have rendered certain traits
adaptive and others nonadaptive, and the effects on current
behavior of mechanisms that evolved in response to these
ancestral conditions. Early ethologists, such as Konrad Lorenz, believed that identifying behavioral homologies, similarities between two species explained by their descent from a
common ancestor, was essential for understanding the evolutionary significance of behaviors. The wings of birds and
bats, for example, are said to be homologous as forelegs
because their evolution can be traced to forelegs in a common
reptile ancestor.
Although the evolutionary origin of a behavior must always
be considered, understanding the evolutionary significance of
a behavior does not depend on tracing its phylogenetic origin,
nor does it depend on discovering analogies, similarities in
two or more species, such as the wings of birds and insects,
that are due to their response to similar ecological pressures
rather than their common ancestory. Although Thornhill and
Thornhill (1987) in an evolutionary analysis of rape in scorpionflies and humans suggest some similarities in the evolution of this behavior in the two species, they do not claim
that human rape is analogous to scorpionfly rape or that
understanding scorpionfly rape helps explain human rape.
They do argue that the same theory, the theory of evolution,
can help understand the behavior of both species.
Understanding the evolutionary significance of a behavior
requires distinguishing functions from beneficial effects (Williams, 1966). Beneficial effect refers to an attribute that currently contributes to fitness but that did not necessarily contribute to fitness in an ancestral population. Function refers
to an effect that was the basis of differential reproduction in
an ancestral population. A knowledge of an adaptation's
function tells us about the solution that natural selection
designed for dealing with a problem that our ancestors encountered in their environment. Although an adaptation may
currently have many beneficial effects, it must have had at
least one function. Although writing is a beneficial effect of
the anatomy of the human hand, it is not a function of the
hand because differential reproduction with respect to writing
did not occur when the human hand was evolving its present
form.
Consider a hypothetical example designed to illustrate the
importance of this distinction. Under current environmental
conditions, grief at the death of an offspring contributes to
current lifetime inclusive fitness through three beneficial effects: (a) eliciting help and support for the griever from close
associates, (b) concentrating the care and attention of the
griever on the offspring of close relatives, and (c) causing
sexual arousal, which increases the likelihood of replacing the
lost offspring (David, 1972). Now further suppose that elicit-
17
ing help from close associates was the only effect contributing
to fitness in the ancestral population and that the other two
effects are due to differences between the ancestral and current
environments. On the assumption that the emotion of grief
evolved to elicit help from close associates rather than because
of the other two putative effects, providing support similar to
that received from ancestral close associates is likely to be a
better therapy than focusing attention on relative's children
or producing sexual arousal in the griever. This example
illustrates that an understanding of an adaptation for dealing
with environmental conditions encountered by our ancestors
can help us to understand how current environmental conditions affect current behavior.
Studying the evolutionary significance of a human behavior
is not simple. Looking for analogies or homologies is difficult
because other species of Homo are extinct. Evaluating the
fitness consequences of current behavior is not a reliable guide
because of the massive changes in human environments during the last 30,000 years. Moreover, these traditional evolutionary methods do not provide the fine-grained data that
interest psychologists.
The predictive method, which uses a variety of contemporary data to test predictions derived from evolutionary theory
(Barash, 1982), seems to hold the most promise for studying
the evolutionary significance of human behaviors because it
does not require tracing the history of the behavior in the
fossil record, comparing human behavior with that of other
species, or assuming that the behavior in question is currently
adaptive. When it is applied to human behavior, I believe it
involves at least three processes: (a) developing quantitative
models to explore the strongest prediction that any hypothesis
makes about a putative behavioral adaptation—that the hypothesized behavior actually could have increased fitness
under ancestral ecological conditions; (b) developing predictions about current behavior from two or more competing
hypotheses and using a variety of data to test them; and (c)
studying putative evolved, proximate psychological mechanisms associated with the behavior. Consider how these processes can be used in studying Surbey's (1987) suggestion that
anorexia nervosa might be the result of an ancestral adaptation for suppressing reproduction when environmental conditions are not conducive to the survival of offspring.
Anorexia nervosa is a syndrome characterized by a relentless pursuit of a thin body shape and an exaggerated dread of
fat and weight gain. The vast majority of cases are found in
adolescent females, and emaciation and amenorrhea are essential features. Body image distortions, hypothalamic and
pituitary dysfunction, sociocultural factors, and family patterns are associated with the disorder. For discussion of this
syndrome, see Garfinkel and Garner (1982).
I consider four of the many hypotheses for explaining this
disorder (see Table 2). Two are competing evolutionary explanations, and two are nonevolutionary explanations. The
example, taken from our research, is part of a more elaborate
research design that compares predictions made from three
evolutionary and seven nonevolutionary hypotheses. First,
consider the evolutionary hypotheses. Imagine an ancestral
population of protohumans, assumed for the sake of argument
to be living in small groups characterized by hierarchical
CHARLES B. CRAWFORD
18
Table 2
Evolutionary and Nonevolutionary Predictions About Anorexia Nervosa
Predictions
Socioeconomic status (SES)
of family
Incidence positively correlated
with SES of family because of
pressure for high-status girls
not to marry low-status boys.
Age of onset
Peaks in incidence around puberty and at times of high female competition (e.g., entering a new school or job).
Sex of anorexic
Female.
Inclusive fitness"
Anorexia increases the resources available to close relatives, improving their survival
and hence the inclusive fitness
of the anorexic.
Incidence negatively correlated
with SES because low-status
families have restricted resources.
Incidence increased in those of
low reproductive value (i.e., in
the very young and the very
old).
Male or female; boys will have
a greater tendency to become
anorexic, because girls are a
better bet for parents in difficult situations (Trivers & Willard, 1973).
Developmental psychobiologyb
Anorexia results from fear of
attaining adult weight and sexual function.
No relation, unless age at puberty is related to SES.
Onset from puberty to age
when other systems mature
enough so that adult body
shape is no longer feared.
Female.
Socioculturalb
Anorexia is an attempt to conform to the cultural ideal that
thin women are beautiful.
No relation, unless concern for
appearance is related to SES.
Hypothesis
Female-Female Competition"
Anorexia serves to delay reproduction in situations involving
severe female-female competition.
Onset around puberty. Inci- Female or homosexual male.
dence in later years depends on
concern for beauty of women
in different social situations;
for example, the incidence of
anorexia should decrease
among older married women
but may remain high among
those still seeking husbands.
" Evolutionary hypothesis that requires assumptions about behavioral adaptations in ancestral populations, ' Nonevolutionary hypothesis that
requires no assumptions about behavioral adaptations in ancestral populations.
social organization of both males and females. High-status
males might offer their offspring better alleles and better
resources. The female hierarchy could control access to resources needed for successful child rearing and high-status
males.
In such groups adolescent females lacking social and physical skills might have trouble dealing with (a) sexual attention
from peripheral males, and (b) female-female competition
for access to resources required for child rearing. The reversal
of pubertal changes (achieved through moderate weight loss
in lean girls) could eliminate male attention and reduce the
intensity of female-female competition, giving these females
time to develop social and physical skills. Such adaptive
reproductive suppression has been documented in many species by Wasser and Barash (1983).
Adaptations are often not ideally engineered but instead
are improvised by natural selection from an assortment of
preexisting anatomical structures, physiological processes, and
behavior patterns (Gould, 1982). Thus, the anorexia nervosa
syndrome could be the expression in exaggerated form, altered
because of changed culture and ecology, of an adaptation for
dealing with female-female competition or inappropriate
male attention. The body image distortions, desire for thin
physique, and hypothalmic and pituitary dysfunctions frequently causing extreme weight loss in our culture may be
remnants of the preadaptations that natural selection put
together to produce more moderate weight loss in protohuman adolescent females.
The high levels of female-female competition and inappropriate male attention characteristic of some urban settings,
such as the large city high school, may result in a malfunctioning of the adaptation that was fine-tuned to the environment of the ancestral population, producing the psychiatric
anorexia nervosa syndrome. Viewing this syndrome from the
perspective of ultimate causation not only enables us to
understand the multiple proximate causal factors associated
with its onset but also suggests that the training of social and
physical skills, rather than behavior modification or drug
therapy, ought to be the treatment of choice.
The first phase of our work involves developing quantitative
models for the evolutionary hypotheses to explore their consequences for fitness in an ancestral population. An extension
of Wasser and Barash's (1983) reproductive suppression
model to situations of high female competition and early
menarche shows that even pathological anorexia nervosa, with
its high fitness costs, might result in a net increase in fitness
if it delayed first reproduction of hunter-gatherer women
when the probability of reproductive success was low but
expected to improve (Anderson & Crawford, 1988).
From the second phase, Table 2 shows some of the many
predictions made from the assumption that anorexia nervosia
is related to an adaptation for dealing with excessive female-
EVOLUTION, RELEVANCE, PSYCHOLOGY
female competition (Crawford & Anderson, 1988). In addition, Table 2 includes predictions from an alternative evolutionary hypothesis that food refusal could be a form of altruism directed to close relatives, increasing the inclusive fitness
for the fasting person. Note that predictions from the evolutionary hypotheses do not require interspecies comparisons,
the tracing of evolutionary history, or the assumption that
anorexia nervosa is adaptive in modern societies.
Table 2 also includes two nonevolutionary hypotheses from
the psychological literature. The sociocultural hypothesis
(Garfinkel & Garner, 1982) states that anorexic behavior is a
consequence of the desire for a thin body form because of
cultural pressure. The developmental psychobiology hypothesis (Crisp, 1967) states that anorexia nervosa is caused by the
girl's fear of attaining adult weight and sexual function.
Consider the pairwise comparisons of the predictions about
socioeconomic status (SES) of the family, age of onset, and
sex of anorexics in Table 2. Because the two evolutionary
hypotheses, female-female competition and altruism, lead to
different predictions for all three variables, data can be obtained to exclude one of them. Note that these three variables
allow a clear decision between the inclusive fitness hypothesis
and both of the nonevolutionary hypotheses. However, given
the three variables, it is not possible to distinguish between
the female-female competition evolutionary hypothesis and
either of the nonevolutionary hypotheses, as they make different predictions for only one variable, SES of the family.
Similarity, it is not possible to distinguish between the two
nonevolutionary hypotheses because they do not make
sharply different predictions about the SES of the family and
age and sex of anorexics. Adding variables and making additional predictions are necessary to distinguish between these
hypotheses.
For example, the two nonevolutionary hypotheses make
different predictions about prognosis. The developmental psychobiology hypothesis predicts recovery once intellectual and
social maturity have occurred, whereas the sociocultural hypothesis predicts that recovery does not occur until the girl
no longer perceives the pressure to conform to current standards of beauty.
Finally, an evolutionary explanation can be strengthened
by showing that proximate psychological mechanisms operate
in a way that does not violate the principle of adaptive design.
For example, if anorexia nervosa is a consequence of an
ancestral adaptation for reproductive suppression, it ought to
be possible to make predictions about preferences for different
types of food, given that the function of the weight loss is to
stop reproduction while still maintaining the energy for continued physical and psychological development. It may also
be possible to use the methods devised by Cosmides (1985)
for studying the structure of the cognitive processes in anorexic patients.
This example can be used to make a number of points.
First, it reveals how a consideration of the ultimate causes of
a behavior can enhance our understanding of the proximate
factors responsible for its development. Second, it illustrates
the wide variety of data that can be used to test hypotheses
derived from evolutionary theory. Third, it helps clarify the
role that the social environment can play in producing a
19
behavior disorder. Fourth, it emphasizes that determining
whether a behavior is an adaptation or merely a beneficial
effect of an adaptation is a difficult task. Fifth, it illustrates
how hypothesis testing, ecological modeling, and exploring
the naturally selected features of cognitive processes can contribute to the understanding of a current behavior. Finally, it
shows how a consideration of the evolutionary significance of
a behavior can give us valuable insights into its origin and
development and hence into environmental interventions that
might alter the behavior.
This example ought not to be taken to imply that all human
pathologies can be traced to putative adaptations in ancestral
populations. Any environmental change has the potential of
disrupting the biological and social functioning of an organism and reducing the level of adaptedness. If the environment change is small or preadaptations leading to a new
adaptation are available, natural selection may in time enable
the organism to adjust to the new environment. However, if
the change is large or relevant preadaptations are not available, the new environment may produce great stress for the
organism and may even result in extinction of the species.
During the last 30,000 years, human inventions ranging
from agriculture to television have irreversibly changed human culture. Each of the changes may lead to stress and
pathology. It will be possible to trace some of these pathologies
to the overloading and distortion of ancestral adaptations.
However, there will be others that do not have an evolutionary
history. For example, the change from a hunter-gatherer to
an agricultural way of life may have resulted in changes in
behavior that reduced the level of exercise. If the level of
physical activity in ancestral humans was involved in some
aspect of metabolism, its reduction in modern humans may
result in a slight biochemical dysfunction that may lead to a
physical or behavioral pathology. Such a pathology does not
have an evolutionary history in the strict sense because it
cannot be traced to an adaptation for dealing with ancestral
environmental conditions.
Conclusions
Psychologists must pay attention to the theory of evolution
for a number of reasons. It can help them to eliminate
outmoded concepts from their thinking and can provide a
rich source of ideas for developing explanations for behaviors
that cannot readily be explained with current psychological
theories. Because the behavior of individuals is the "evolutionary pacemaker," psychology, which is the study of the
behavior of individuals, must be a key discipline for understanding the evolution of human nature. An evolutionary
perspective focuses attention on the mechanisms that have
evolved to enable organisms to deal with varying conditions
in their environment. As such it can provide insights into the
development of both normal and abnormal behaviors and
possible environmental interventions that may be helpful in
preventing or changing undesirable behavior. However, the
most important benefit to accrue to psychologists from a
study of evolution is that it can broaden the understanding of
the causes of behavior and help relate psychology to the other
social and life sciences.
CHARLES B. CRAWFORD
20
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Received March 22, 1988
Revision received August 26, 1988
Accepted August 29, 1988