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EVOLUTIONARY
PSYCHOLOGY HUMAN
REPRODUCTIVE
BEHAVIOUR
• Intersexual selection (‘Inter’ =
‘between’ the sexes). Females
‘chose’ males. Females have
less reproductive potential and
so it is important to make the
right selection.
Intrasexual selection
(‘Intra’ = ‘within’ the
same sex). Males
compete with each
other to be chosen by
females – by impressing
or charming them.
•
•
Buss (1995) – Males can
produce lots of sperm for
much of their life, but
females make a greater
investment in offspring
(few eggs in a restricted
period of life, gestation
and infant care).
Because of this females
chose males that are able
and/or willing to invest in
their offspring. These males
achieve higher
reproductive success.
Natural Selection Vs
Sexual Selection
• Natural Selection
‘Survival of the fittest’: This
basic evolutionary
principle suggests those
with the most physically
advantageous traits will
survive in their environment
longest and so have most
mating potential.
Natural Selection Vs
Sexual Selection
• Natural Selection
Can you think of
examples from the
animal kingdom where
this is not true, i.e. where
‘attractive’ mates
actually have an
evolutionary
maladaptive trait?
Sexual Selection
PEACOCKS!
Peacocks have such
large, bright plumage
and tails? – This actually
impedes movement,
slows them down and
makes them more
noticeable to predators.
Why are they still
selected?
Sexual Selection
• Proposed by Charles
Darwin - If a male
characteristic enhances
reproductive success it
becomes established as
a preference among
females so males under
pressure to improve on
that characteristic (trait).
• Males with the trait are
preferred and the trait
passed on.
• *Sexy Sons hypothesis
Handicap
Hypothesis Zahavi (1975)
Whilst it is true that certain
characteristics seem
maladaptive, the very fact that
their owners presents them and
is still alive and healthy (and of
reproductive age) indicates
great survival skills. The original
‘good genes’. They’ve
overcome a potential
handicap (in terms of survival)
and so are a ‘good catch’.
These exaggerated
features/characteristics are a
type of ‘fitness indicator’.
Fitness
Indicators
Anything which, not verbally
(animals can’t speak and nor
could early man) which
suggests the presence of
‘good genes’, ‘survival skills’
or ‘parenting potential’
Can you think of any in both
the animal kingdom and in
humans?
What, potentially could be a
problem with human Fitness
Indicators?
Fitness Indicators
The main problem is, Fitness
Indicators can be faked – e.g
red lipstick/collagen/ mascara/
implants/dentures/wigs….
In humans an example of an
‘honest indicator’ is facial
symmetry as it is physiologically
difficult to fake.
Consequences of
Sexual Selection
• Sexual Dimorphism – physical
differences between the sexes.
Human males are generally
larger because it is suggested
humans evolved in a
polygynous mating system
(one male many females) with
more intense sexual
competition between males
than females. The bigger the
males the better they were at
competing!
Consequences of Sexual
Selection
• Facial characteristics
• Research has shown
that females with
neotenous (child-like)
features (large
eyes, small noses) are
universally thought of as
more attractive.
Consequences of Sexual Selection
• Facial characteristics
• Research has shown that
male faces with strong jaws
and large noses
(influenced by the
hormone testosterone) are
more attractive (Thornhill
and Gangestad, 1993)
• Facial characteristics
• Bruce & Young 1998 –
found a preference
for symmetrical faces
• Facial symmetry – free
from harmful
mutations
WHICH BODY SHAPE DO YOU
PREFER?
BODY SHAPE
• Singh: found a
preference for waist to
hip ratio of 0.7. This
produces an hourglass body shape.
• Preference persists
despite a figure being
curvier or slimmer.
Cross-cultural research
•
BUSS studied partner
preferences in 33 different
countries
• 10,000 men & women aged
16-28 were asked about
their preferences.
• In all cultures he found:
1. Men valued physical
attractiveness more than
women
2. Men valued women
younger than themselves
3. Women valued financial
success & ambition more.
PARTNER SELECTION – Lonely
Hearts
• Dunbar used 900 personal
ads from north American
newspapers.
• 42% of men wanted a
younger partner
• 25% of women wanted a
younger partner
• 44% of men sought a
physically attractive woman
• 22% of women sought a
physically attractive man
Cross-cultural research
•
•
•
Why is Cross-cultural
research so important for
Evolutionary psychology?
If similar findings, patterns or
trends are found in different
areas of the world it suggests
a universality in nature, i.e.
being innate and natural to
all humans – supporting E.T.
If there are differences in
behaviour from one place to
another it supports the
influence of culture on
behaviour, challenging E.T.
Mating Strategies
•
•
•
•
Monogamy – mates
with one partner
Polygyny – One man
with multiple women
Polyandry – one
woman with multiple
men
Promiscuity – both
partners with multiple
partners
Which mating strategy makes most evolutionary
sense?
DIFFERENCES IN
REPRODUCTIVE BEHAVIOUR
Men and women’s
behaviour varies:
1. Tendency to engage in
casual sex: men are more
likely to have short-term
relationships and one-night
stands
• Clark and Hatfield: strangers
were approached and
offered
a. to go out that night
b. to go back to their house
c. to have sex with them
Clark and Hatfield - RESULTS
1. Go out that night
2. Go back to their
house
3. Have sex with them
1. 50% of both men
and women
agreed to this
2. 69% of men would
but only 6% of
women would
3. 0% of women
agreed
75% of men agreed
Mating Strategies
Norman and Kenrick
investigated what
the desirable traits
were for men and
women seeking a
one night stand
Norman and Kenrick
• If you were to have a one
night stand, which of these
characteristics would be
most important? You may
only choose 1
• Kindness
• Physical attractiveness
• Vitality
• Generosity
• Humour
Norman and Kenrick
• Norman and Kenrick found
that both men and women
rated physical attractiveness
the most
• They concluded that men
choose this because it
indicates fertility, and women
choose it because it
indicates good genes for
their offspring
Methodological Problems
• What issues may have
arisen in Clark and
Hatfield’s study?
• What issues might Buss
have encountered?
• How truthful do you
think people are when
their sexual habits are
being investigated by
psychologists?
PARENTAL INVESTMENT
THEORY
• Trivers proposed that
differences between
males and females
exist because they
make different
amounts of
investment in the
offspring
MALES
• Males have a large amount of sperm
and remain fertile throughout life.
• The best way to increase his reproductive
success is to have many matings with
multiple partners
FEMALES
• Human females invest much more in each
offspring.
• Eggs are 100x larger than a sperm, and she
only produces one per month.
• Her reproductive life is shorter than a male’s (30
years)
• Her pre- and post natal investment is large
(carrying the foetus for 40 weeks, suing
thousands of calories from her body)
• She gives birth and carries on investing in the
So her best strategy is to ensure survival of
each offspring
•
Buss (1995) –
Hunter-gatherers 10,000+ years ago
were less faithful, as their
social/sexual groups were less rigid.
• Are humans by nature more
promiscuous than monogamous?
•
Baker and Bellis (1995) - 9% fathers
not really the father, 29% in E-poll
in 2003 had admitted to an affair.
These figures suggest that the
majority of humans are not
adulterous, challenging the idea of
promiscuity being part of human
nature
SEXUAL JEALOUSY
• Buss asked M & F students to
imagine their current partner:
• A. having sex with someone else
• B. in love with someone else
• Stress responses were measured
• Men – more distressed by A
• Women – more distressed by B
PARENTAL
INVESTMENT
THEORY
• Indian farmer Nanu Ram Jogi,
fathered his 21st child at the
age of 90
• The oldest recorded instance
of childbirth is a woman of 54
• King Molay the Bloodthirsty
(medieval Morocco) is
recorded as fathering over 800
children.
• The world record for a woman
is 69 (Russian Peasant)
EVALUATION OF PARENTAL
INVESTMENT THEORY
1. It helps to understand mate
preferences. Women seek men with
good genes and who show
commitment to help raise the offspring.
2. This is explained by Buss’s findings. Men
prefer younger women because they
are more fertile
3. It also explains why men engage more
in short-term matings, and women are
reluctant
EVALUATION OF PARENTAL
INVESTMENT THEORY
4.It explains the difference in sexual jealousy.
Why would a man feel more threatened by his
partner’s sexual infidelity?
Why would a woman feel more threatened by
her partner’s emotional infidelity?
5. But it does not explain why some women have
one night stands and some men are faithful.
6. It does not explain the existence of
homosexual and other relationships which are
non-reproductive