Evolutionary explanations of
By the end of the lesson you will be able to (YBWAT)
•Give a definition of parental investment
•Explain using the evolutionary approach why women invest more
into potential offspring
•Explain why men want to avoid cuckoldry (using the Jeremy Kyle
show as an example)
•Evaluate the usefulness of parental investment theory for the
• Trivers defined parental investment (PI) as, “any
investment by the parent in an individual offspring that
increases the offspring’s chance of surviving (and hence
reproductive success) at the cost of the parent’s ability to
invest in other offspring.”
• PI includes the provision of resources (such as food,
energy and time used in obtaining food and maintaining
the home and territory), time spent teaching offspring, and
risks taken to protect young.
Fundamental asymmetry in humans
• As we have previously
discussed, the female egg
cell is much more costly to
produce than a male
• Females must nourish for
the offspring for 9 months,
which means she can only
have a limited number of
Limited offspring for females
• Breastfeeding can last for
up to 4 years in some
societies (Shostak, 1981).
• In the UK and other
Western cultures the
average is between 3 and
• By contrast, a male can
have a virtually unlimited
number of offspring
(provided he can find
females to mate with him!)
• Females must be
• Males compete for
quantity of females,
choose quality male
because of their
Optimum number of offspring?
• Trivers argued that there’s an optimum number of
offspring for each parent. A low-investing male could
afford many offspring and may favour a ‘quantity rather
than quality’ approach.
• Females, on the other hand, would prefer quality rather
than quantity. Consequently, females generally need to be
much more choosy about whom they mate with.
• A consequence of this is that males often want more
children than females. Perhaps because they do not have to
endure labour, childbirth and breastfeeding.
Why do women invest more?
• In common with other
mammals, human females
breastfeed their young, and
so are more burdened by the
extended period of childcare
that results from this
• Human mothers therefore
not only make the greater
prenatal contribution of
pregnancy), but also make
the larger postnatal
contribution as well.
Cuckoldry – and how it relates to
The Jeremy Kyle show
• When males do invest parentally (e.g.
through their resources), they are under
pressure to protect themselves from the
possibility of cuckoldry (i.e. investing in
offspring that are not their own).
• Because human males make a considerable
investment in their children, they have a
greater concern than females about the
fidelity of their mates (Miller, 1998).
• As a result, they try to ensure that their care
is not misdirected towards non-relatives –
e.g. through adultery laws that define the
offence in terms of the woman’s marital
status rather than the man’s.
Sexual and emotional jealousy
• The possibility of sexual infidelity posed different adaptive
problems for males and females.
• A man whose mate was unfaithful risked investing in offspring
that were not his own, whereas a woman whose mate was
unfaithful risked the diversion of resources away from her and
• Sexual jealousy, therefore, may have evolved as a solution to
these problems (Buss, 1995). Men are more jealous of the sexual
act (to avoid cuckoldry) while women are more
• jealous of the shift in emotional focus (and consequent loss of
• The world record for the
number of children is 888,
fathered by Ismail the
an Emperor of Morocco
• A Russian woman gave
birth to 69 children (which
included 16 sets of twins,
7 sets of triplets and 4 sets
Which route to take?
• To be successful, you must pass on your genes. This means,
in most cases, that you must have more than 2 babies who
grow up to do the same.
• So, does it make more sense to ‘sow your wild oats’ with
lots of women, in the hope that at least some of them
mother your children, or to stick to one woman and watch
her like a hawk so you can be sure she’s mothering only
• YOU DECIDE!
AO2: What about seahorses?
In certain species, males provideIn certain species, males provide
a great deal of parental care; fora great deal of parental care; for
example, a male seahorse has aexample, a male seahorse has a
pouch where it keeps the infantpouch where it keeps the infant
until maturity (see pregnant maleuntil maturity (see pregnant male
seahorse in picture). The femalesseahorse in picture). The females
compete with each other for thecompete with each other for the
male’s attention.male’s attention.
Be careful here – the examiner isBe careful here – the examiner is
interested in human behaviour,interested in human behaviour,
not animal behaviour.not animal behaviour.
AO2/AO3: Evaluation of PI theory
• Inconclusive empirical support:
According to Daly and Wilson (1988) children under the age of 2
are at least 60 times more likely to be killed by a step-parent –
almost always a stepfather – than by a natural parent.
This is exactly what evolutionary theory would predict, since step-
parents and stepchildren are genetically unrelated, whereas a
child inherits half its genes from each biological parent.
However, most stepfathers don’t kill or abuse, and a minority of
biological fathers do: these findings are difficult to square with
any explanation based on shared/non-shared genes. (Has
societal and family structure changed significantly since Trivers
published this theory in 1972? SLH)
AO2/AO3: Evaluation of PI theory
• How do evolutionary psychologists explain maternal
• More tricky still for evolutionary theory to explain is the case of
the woman who kills her newborn baby (neonaticide).
• According to Pinker (1997), when such an act takes place in
conditions of poverty, it could be regarded as an adaptationist
• The psychological module that normally induces protectiveness
in mothers in their new-borns is switched off by the challenge of
an impoverished environment.
• This means that both killing and protecting are explained by
evolutionary selection. As Hilary Rose (2000) says, this explain
everything and, therefore, nothing.
AO2: Maternal investment
• There are two consequences of the
high cost of maternal investment.
First, infant dependency means
females want male providers. Second,
the expense of childrearing means
that females want to ensure good
quality offspring so they don’t waste
• One way to achieve this is to marry a
man who has good resources and is
caring, but shop around for good
genes through extramarital affairs
with ‘studs’ – attractive men
advertising good genes but no
• Although accurate data for
mistaken paternity are
notoriously elusive, there is
some evidence from a magazine
survey of over 2700 UK women.
From the results of this survey,
Baker and Bellis (1990)
estimated that as many as 14%
of the population were products
of extra-marital matings.
AO2: Evidence for sex differences in
• In line with PI theory predictions about sex differences
in type of jealousy, Buss et al. (1992) found that male US
students indicated more concern about sexual infidelity,
whereas female students expressed more concern about
• This was supported by physiological responses when
respondents asked to imagine scenes of sexual or
emotional infidelity – the men showed much more
distress for sexual than emotional infidelity.
• This research evidence supports the idea of jealousy
differences between males and females.
AO2: Shared parental care
• In humans, joint parental
care is desirable because of
the high costs of successful
• In any situation where males
can increase the success of
childrearing, it will pay them
to do so (Dunbar, 1995).
• This means that in humans,
males do restrict their
and invest more in each
AO2: SEX DIFFERENCES IN PARENTAL
• In 2008, Conservative MP Michael Gove claimed that ‘lads’ mags’ such as Zoo and
Nuts reinforced a ‘shallow approach’ to women, and linked them to a rise in feckless
fatherhood and family breakdown. Could this be the case, or might male attitudes to
parenting actually be shaped more by biological forces (as predicted by PI theory)?
• Geher et al. (2007) studied 91 non-parent heterosexual undergraduates. Each
completed a parental investment perception scale, which included statements such
as: ‘I believe that I am very prepared to raise a child at this time in my life’. They were
additionally exposed to various parenting related scenarios such as, ‘You are the
parent of a three-year-old girl who has an ear infection. Your plans for the day have
completely changed, as you now have to look after her’.
• Although there were no sex differences in self-report responses to parenting on the
parental investment perception scale, there were clear differences in ANS arousal to
the different parenting scenarios. Males showed significantly increased heart rate
when presented with scenarios that emphasised the costs of parenting (e.g. that
they would be unable to work). Researchers concluded that, consistent with
predictions from PI theory, males are biologically less prepared than females to
confront issues associated with parenting.
AO2: AN EVOLUTIONARY
• According to PI theory, parents maximise their reproductive fitness by gradually
withdrawing their investment from older children in favour of younger siblings.
• First and last-born siblings hold a privileged status with respect to parental
investment. First-born siblings are closer to their reproductive age when they will
pass their genes to the next generation. Last-born siblings also have privileged status
because they need more investment than older siblings.
• Parents, therefore, are more easily pressured into investing in their first and last-
born children than they are in their middle-born children. This hypothesis is
supported by Andrews (2006), who analysed responses from a survey of 1600 US
adolescents, and found that severe suicide attempts were significantly more common
among middle-born compared to first-born and last-born children.
• Middle-born children would need to make such risky attempts in order to extort
increased investment from their parents. This study supports the view that suicidal
behaviour may well be an adaptive response in line with the predictions of PI theory.
Plenary and Homework
• Can you:
– Give a definition of
– Explain using the
why women invest more
into potential offspring?
– Explain why men want
to avoid cuckoldry
(using the Jeremy Kyle
show as an example)?
– Evaluate the usefulness
of parental investment
theory for the modern
• ‘To put it at its most basic, women want
resources and men want to spread their
genes.’ Discuss the evolutionary
approach to explaining parental
investment in humans (eg sex
differences). (24 marks)
• This question was last asked in January
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ANSWER FOR THIS QUESTION
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• Deadline: Monday 30th