Relationships N0. 5

Parental Investment ( PI)

Trivers 1972 – Parental Investment Theory

• “Any investment by a parent in an offspring that increases the chance that the offspring will survive at the expense of that parent’s ability to invest in any other offspring (alive or yet to be born.”)

Unequal Parental Investment • In most species males and females do not invest

equally in their offspring. Females investment is greater because female eggs are less numerous and more “costly” to produce than male sperm. A female can only have a limited number of offspring whereas in theory, a male can have an unlimited number.

• As a result of this biological inequity, females must be much choosier concerning potential mates. Males compete for quantity of females, whereas females select for quality of males and their resources.

Sex Differences in Parental Investment

• The most obvious difference is that human males can opt out of parental investment in a way that females cannot.

• Daly and Wilson 1978 –: By expending a relatively large part of their reproductive effort on courtship and mating, males of most species can afford to devote rather little in comparison, to parental care.

• HA!!

How do Human Females invest More?• Human brain size increased in response to adaptive

pressures among our ancestors. This resulted in more difficult childbirth because of the enlarged skull. So to compensate for this, childbirth in humans occurs earlier in development which means that human infants are born relatively immature compared to other animals.

• Like other mammals, human females breastfeed their young and so are burdened by the extended period of childcare that results from this prolonged immaturity. SO – human mothers make a greater prenatal contribution of resources (long pregnancy, dangerous labour) and a larger postnatal contribution too.

The Costs of Maternal Investment• For Our females ancestors, the minimum parental

investment would have been a nine-month pregnancy followed by years of feeding, carrying and caring.

• For Our male ancestors, (and for some irresponsible 21st century males!!) the minimum parental investment would have been a few moments copulation and a teaspoonful of semen.

• This means that there is an enormous difference in the potential reproductive success of the sexes, making random mating for females very costly.

Commentary on Maternal Investment• There are Two consequences for this high

maternal investment.

• 1. Females need male providers.

• 2. The huge expense of child rearing (in terms of time and effort) means that females want to ensure good quality offspring. – But how is this achieved with maximum effectiveness?

• WELL!! This is interesting!!!

One Way!• Is to marry a man who has good resources and is

caring. BUT THEN• Shop around for good genes though extramarital

affairs with “studs” – attractive men “advertising” good genes but no resources. Then you have the best of both worlds from an evolutionary point of view.

• In a magazine survey of over 2,700 UK women, Baker and Bellis, 1990, estimated that as many as 14% of the population were products of extramarital mating.

Paternal Investment and Cuckoldry• When males do invest parentally, for example

through their resources, they are under pressure to protect themselves from the possibility of cuckoldry. (investing in offspring who are not their own)

• For this reason, according to Miller 1998 men have a greater concern than females, about the fidelity of their mates. Adultery laws, (traditionally drafted by males) have been harsher on women than on men, and even today in some parts of the world, a woman’s adultery is punishable by death.

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. • A woman whose mate was unfaithful risked losing

the man’s resources for her and her family.• Buss 1995 said this is why sexual jealousy evolved.• Men are jealous of their mate having sex with

someone else (because of cuckoldry) whereas:-• Women are more jealous of a perceived shift in their

mate’s emotional focus (because of consequent loss of resources)

Commentary: Evidence for Sex Differences in Commentary: Evidence for Sex Differences in

JealousyJealousy • Buss et al 1992 found that US male students indicated

more concern about female sexual infidelity , whereas female students expressed more concern over male emotional infidelity.

• This was supported by physiological responses when students were asked to imagine scenes of sexual and emotional fidelity.

Shared Parenting – Dunbar 1995 suggests that it pays male humans to restrict their reproductive opportunities and invest more in each individual offspring as this can increase the success of childrearing. And joint parental care is of evolutionary advantage because of the high costs of successful reproduction.

Research Study: Sex Differences in Parental Investment

• In 2008, Conservative M.P Michael Gove claimed that “lad mags” such as Zoo and Nuts encouraged a shallow attitude to women and linked such mags to a rise in feckless fatherhood and family breakdown. Is this true or are male attitudes to parenting shaped more by biological forces as predicted by the Parental Investment Theory?

Geher et al 2007• Studied 91 non-parent heterosexual

undergraduates.

• Each completed a parental investment perception scale after being 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”.

Findings of Geher et al 2007• There were no gender differences in the self-report

responses on the P.I perception scale BUT• There were clear gender 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 the PI theory, males are biologically less prepared than females to confront issues associated with parenting.

Parent – Offspring Conflict• So far we’ve looked at the idea of parental

investment as if the offspring were completely passive in the process. But this is not the case.

• The resource allocations that would maximise parental fitness are not necessarily identical to those that would maximise offspring fitness.

Trivers 1974 argues that this results in the following predictions:

• Parents and their children will be in conflict about when the child should be weaned, with the parents generally wanting to wean the child sooner to free themselves of this time-consuming and costly activity and the child wanting to receive milk for longer in order to ensure healthy growth.

• Parents encourage children to value siblings more than they are naturally inclined to as friendly siblings will share resources, ensuring that all grow healthily with a better chance that the parent’s genes are perpetuated, and freeing the parents to invest in younger and therefore more needy siblings . But in reality siblings are competing for resources.

An Evolutionary Perspective• According to the 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 borns are closer to reproductive age, when their (and the parents’ genes) will be perpetuated

• Last-borns need more caring for as they compete for resources amongst the older, stronger siblings.

This hypothesis is supported by Andrews 2006

• Analysed responses from a survey of 1600 US adolescents.

• Results showed that severe suicide attempts were significantly more common among middle-born compared to first and last-born children.

• This study supports the view that that suicidal behaviour may be an adaptive response in line with the predictions of the PI theory and that middle-borns would make risky suicide attempts in an effort to extort increased investment from parents.

More Parent – Offspring Conflict• Conflict Conflict BeforeBefore Birth Birth – Some pregnant

women suffer from a potentially fatal condition – Pre-eclamsia, which is a dangerous rise in the mother’s blood pressure caused by the foetus secreting hormones in an effort to get more nutrition. It serves the foetus well as it does get more nutrients and is an adaptive mechanism which benefits the foetus at the expense of the mother.

Commentary - Conflict Conflict BeforeBefore Birth Birth • Research has shown that mothers who do have

high blood pressure during pregnancy tend to have fewer spontaneous abortions and larger babies at birth.

• So this suggests that high blood pressure is associated with more healthy foetuses and so is an adaptive strategy for the child. A risky one though, as pre-eclampsia can kill pregnant woman therefore putting the foetus at risk.

More Parent – Offspring Conflict• Conflict Conflict AfterAfter Birth Birth – – When their offspring are

young and helpless it is in the interests of the parents to invest more of their resources in these offspring and less in their older, stronger brothers and sisters.

• When these babies become older and stronger, parents can maximise their own reproductive fitness by directing their limited resources towards the younger offspring and away from the older child.

• Parent-offspring conflict is often most intense at this transfer stage, as the older children attempt to prolong the parents’ primary focus on them for as long as possible.

Commentary - Conflict Conflict AfterAfter Birth Birth • Salmon and Daly 1998 argue that many younger

children don’t bother to compete with their older siblings but instead, opt out of competition for parental attention altogether and develop traits such as cooperativeness to help them form alliances with non-relatives, which could be a useful evolutionary strategy .

• And younger children do tend to be more cooperative. But Shaffer 1993 suggests that this is learnt through having to negotiate with older siblings.

More Parent – Offspring Conflict

• Sibling RivalrySibling Rivalry – – In most sexually reproducing species including our own, offspring will want more than their “fair share” at the expense of their siblings in order to maximise their own fitness.

• As a result sibling rivalries develop as children compete for the attention and resources of parents.

• Can you think of any advantages/disadvantages of being an only-child?

Commentary - Sibling RivalrySibling Rivalry

• Lalumière et al 1996 suggest that a powerful parental strategy to cope with sibling rivalry is to steer siblings along different paths, maximising each individual’s strengths, so that there is less sibling competition and also less subsequent competition for the same mates.

-------------------------------------------------------------• HomeworkHomework – Discuss evolutionary explanations

for sex differences in parental investment and parent-offspring conflict.

• 9 marks + 16 marks