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  1. 1. Evolutionary explanations of parental investment: “Mummy’s babies, Daddy’s maybes” 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 modern family
  2. 2. Trivers (1972) • 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.
  3. 3. Fundamental asymmetry in humans • As we have previously discussed, the female egg cell is much more costly to produce than a male sperm cell. • Females must nourish for the offspring for 9 months, which means she can only have a limited number of offspring.
  4. 4. 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 12 months. • By contrast, a male can have a virtually unlimited number of offspring (provided he can find females to mate with him!)
  5. 5. Biological inequity • Females must be choosier concerning potential mates. • Males compete for quantity of females, whereas females choose quality male because of their resources.
  6. 6. 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.
  7. 7. 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 prolonged immaturity. • Human mothers therefore not only make the greater prenatal contribution of resources (through pregnancy), but also make the larger postnatal contribution as well.
  8. 8. 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.
  9. 9. 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 the family. • 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 resources).
  10. 10. World records! • The world record for the number of children is 888, fathered by Ismail the Bloodthirsty (1672-1727), 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 of quadruplets).
  11. 11. 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 your children? • YOU DECIDE!
  12. 12. 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.
  13. 13. 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)
  14. 14. AO2/AO3: Evaluation of PI theory • How do evolutionary psychologists explain maternal neonaticide? • 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 response. • 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.
  15. 15. 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 their efforts. • 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 resources. • 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.
  16. 16. AO2: Evidence for sex differences in jealousy • 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 emotional infidelity. • 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.
  17. 17. AO2: Shared parental care • In humans, joint parental care is desirable because of the high costs of successful reproduction. • 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 reproductive opportunities and invest more in each individual offspring.
  18. 18. AO2: SEX DIFFERENCES IN PARENTAL INVESTMENT • 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.
  19. 19. AO2: AN EVOLUTIONARY PERSPECTIVE • 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.
  20. 20. Plenary and Homework • Can you: – 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 modern family? • ‘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 2010. • YOU MUST HAVE AN EXAMPLE ANSWER FOR THIS QUESTION • Email to: • Deadline: Monday 30th September 2013