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Evolutionary Explanations For
Human Behaviour
Sexual Selection
• Reproductive success is the basis of the
evolutionary process. Among early humans,
and for us today, those who failed to mate,
failed to pass on their genes. For those who did,
the process involved selecting the right mate by
out-competing rivals, mating at a time which
would ensure a child was conceived and then
behaving in a way that would ensure the child
would survive long enough to pass on its genes
too. It follows then, that we, as modern-day
humans also have a repertoire of psychological
behaviours which give the best possible chance
for our genes to be passed on.
The Nature of Sexual Selection
• In most sexually reproducing species the males
are much more brightly coloured than the
females even when you would think that this
was a disadvantage. (Think of the peacock’s tail
and it’s pathetic flying ability!) Clearly then,
these features enhance reproductive success.
• Charles Darwin (1874) came up with the theory
of sexual selection and the two processes
through which it took place.
Natural Selection V Sexual
Selection
• Natural Selection – Darwin – Survival
of the fittest – BUT – how does this
explain why Peacocks have such large,
bright plumage and tails? – Wouldn’t
they be more noticeable and slower.
Intrasexual Selection (mate competition)
• Members of one sex (usually males) compete
with each other for access to members of the
opposite sex. The winners mate and so pass
on their genes, whereas the losers do not. So
whatever traits lead to successful mating in
these contests will be passed on to the next
generation.
Intersexual Selection (mate choice)
• This involves the preferences of one sex for
certain traits in the opposite sex. For example, if
females prefer tall males, over time there would
be an increase in the number of tall males in the
population. So therefore the preferences of one
sex determine the areas in which the opposite
sex must compete. It may be plumage in
peacocks and economic resources in humans. So
these indicators (be they plumage, tallness etc )
become signals to the opposite sex that the
bird/animal/human has “good genes” likely a) to
produce successful offspring b) to be able to
give protection and support to offspring.
Commentary
• The logic of sexual selection – why have
mechanisms for mate choice evolved? After all
choosiness requires time and energy. It is because
it pays to be choosy because the genetic quality of
a mate will determine the genetic quality of the
offspring. Remember, reproductive success is at
the very heart of the evolutionary process – low
quality mates (those who are unattractive and
unhealthy will be more likely to produce
unattractive, unhealthy offspring. By joining
forces with an attractive, high-quality mate, the
offspring are higher quality and so the individual’s
genes are much more likely to be passed on.
• Sexual attractiveness is determined by
characteristics that suggest that the person
(male or female) is a ‘safe genetic bet’
• Fashion, in evolutionary terms, is a ‘con
trick’ to suggest or exaggerate favourable
characteristics
www.psychlotron.org.uk
Sexual selection & fashion
Sexual selection & fashion
©2005 University of Utah
www.psychlotron.org.uk
Bustle Dress
Sexual selection & fashion
©2006 Suavecito Apparel Co.
www.psychlotron.org.uk
Zoot Suit
Sexual selection & fashion
©1984 Korova Records
www.psychlotron.org.uk
Rara Skirt
Sexual selection & fashion
©2006 The Charleston Museum
www.psychlotron.org.uk
Corset
Sexual selection & fashion
©1995 BBC
www.psychlotron.org.uk
Cutaway Coat
Sexual selection & fashion
©2006 LeMor
www.psychlotron.org.uk
Eyeliner
Short-term Mating Preferences
• Men appear to have a number of evolutionary
strategies with regard to sex, one of which has
evolved for short-term mating success and is based
on the Parental Investment Theory, which we come
to in detail in the next chapter. The idea is that
because men are competing for quantity of females
in order to pass on more copies of their genes than
a less successful rival, they would want to have sex
earlier in a relationship. The less time a man waits
before having sex , the larger the number of women
he can impregnate in a given time. Women
however, are not under this evolutionary pressure
as no matter how many men they have sex with,
there can only have one pregnancy at a time.
Issue – Gender Bias in Sort-Term Mating
• But – It takes two to tango! Every time a man
has sex with a willing partner the woman is also
having sex with a new partner so men’s greater
willingness to have sex with multiple partners
could not have evolved without willing females.
• Clearly there must be some benefits for women.
• Greiling and Buss 2000, suggest that women
profit in many ways, including using short-term
mating as a way of leaving poor-quality
relationships or as a way of producing more
genetically diverse offspring.
More on Short-term Mating Preferences
• According to Buss and Schmitt, 1993, men “lower
their standards” regarding short-term mating
opportunities and show a marked decrease in
attraction following sex. The researchers argue that
this is an evolved adaptation to bring about a hasty
departure which prevents them spending to much
time with a woman. HA!!!!!!
• “So little time, so many women to impregnate”(RN)
• And don’t’ quote me!!!!
Research – Short-Term Mating Strategies
• Clarke and Hatfield 1989 – Students on a college
campus were approached by men and women
experimenters who said:• “Hi, I’ve been noticing you around campus and I
find you very attractive”.
• The students were then asked:
• 1. Would you go on a date with me?
• 2. Would you go back to my apartment with me?
• 3. Would you have sex with me?
The subtle approach then!!
This is what happened
• Females – 50% agreed to go on a date
•
6% agreed to go back to his flat
•
0% agreed to have sex
• Males - 50% agreed to go on a date
•
69% agreed to go back to her flat
•
75% agreed to have sex
• These results gave been replicated in other studies and seem
to provide compelling evidence that men have evolved
psychological mechanisms to ensure success in short-term
mating – these include a) a desire for sexual variety, b) the
tendency to let little time elapse before seeking sex and c) a
willingness to consent to sex with strangers.
Long-term Mating Preferences
• Since parental investment is high in longterm mating, sexual selection would predict
that both men and women would be choosy.
Poor long-term mate choice would mean
wasted resources and opportunities.
According to Buss 2003 Woman would be attracted to
males who:-
a) are able to invest resources in her and the children
b) are able to physically protect her and the children
c) show signs that they’ll be a good parent
d) are compatible enough to ensure minimal costs to
her and the children.
And what about the men?
Well in order not to give away resources
indiscriminately, men would be most attracted to
females who display signs of fertility – an indication
of their reproductive value. The following research
by Buss found universal trends in men and women .
Research: Sex Differences in Human Mate
Preferences. Buss 1989
• Study involving 10,000 people from 37 widely
diverse cultures. The following was found.
• Women desired men with financial ambition.(linked
to the ability to support a family)
• Men were more concerned with physical
attractiveness and mates who were younger than
them. (Both potential cues of fertility and
reproductive value)
• Both sexes desired intelligence, kindness and
dependability – all of which are linked to the
qualities needed for long-term investment in
children.
Commentary: Male Preferences for Younger
Women.
• One of the most striking conclusions of Buss’s study
was that men have a distinct preference for younger
women, which is a finding consistent with the theory of
Sexual Selection because the younger the woman, the
greater the fertility. BUT Some critics have tried to
explain this preference in terms of social power –
Younger women are easier to control, and therefore are
preferred as mates. However Kenrick et al 1996
found that teenage males were most attracted to women
who were 5 years older than them despite the fact that
these women showed no interest in them and were
clearly not more easily controlled by adolescent males!
Research Methodology:
Mate-Choice in Real Life
• Do studies such as Buss’s suffer from a lack of
Validity? - i.e. they may indicate preferences
but they’re not a reflection of what happens in
real life. Apparently a study by Buss 1989 of
actual marriages in 29 cultures backs it up. This
research confirms that men do choose younger
women. Not only that, but men who divorce
and remarry tend to marry women who are
increasingly younger than they are.
Research: The Importance of Fertility
Miller et al 2007
Clearly fertility is very important in mate choice,
but how does a male know when a female is “in
oestrus” – fertile? Most female mammals give
out visual or chemical signals which encourage
the male to mate. However in human females
oestrus is apparently hidden, but this study
indicates that woman are most attractive to men
at the most fertile point of their menstrual cycle.
Researchers calculated the tips earned by lap
dancers at varying stages of their cycle. Those
who were in the most fertile part of their cycle
earned almost twice the value of tips compared to
girls who were not.
Commentary: Mate choice and the
Menstrual Cycle
• Interestingly, according to research by PentonVoak et al 1999, female choice of a mate varies
according to her stage in her cycle.
• Woman chose a more feminine version of a male
face as most attractive for a long-term
relationship.
• But during the high conception stage of her cycle
the preferred shape of face was more
masculinised.
What does this tell us about mate choice?
• Sexual selection my well have favoured females
who pursued a “mix mating” strategy.
• A female might choose a partner whose feminised
appearance suggests kindness and cooperation for
parental care –
• But she might also copulate with a male of more
masculine appearance when conception is most
likely because he probably has higher levels of
the sex hormone testosterone. This dampens the
immune system so if he’s well despite this, he
must have a very robust immune system – a very
valuable characteristic to pass on to offspring!
Homework
• Discuss the relationship between sexual
selection and human reproductive behaviour
• 9 marks + 16 marks