Black: AO1 - Description
Blue: AO2 - Evaluation - studies
Red: AO2 - Evaluation - evaluative points/IDAs
Purple: My notes/hints/tips
Parental investment is the dedication of resources, time, and effort towards a child that increases its chance of survival and reproduction, at the expense of parents' ability to invest in other children.
Parental Investment Theory suggests that the differences between males and females in the amount of parental investment are a result of the sex differences in parental certainty. Due to giving birth to it, a female will always have parental certainty over their child, and will therefore be willing to invest more time and resources into ensuring its survival. Because of this parental certainty, a sexual strategy of choosiness rather than promiscuity is more beneficial for women - ensuring that a mate is of good genetic quality and is able to provide resources for the child. Males do not have parental certainty, so therefore will less willing to invest time and resources when they could unwittingly be raising another man's child. Because of this lack of certainty, a sexual strategy of promiscuity is most adaptive for men - having as many children with as many women as possible to increase the chance of having a legitimate child survive.
A female's parental investment in each reproduction is much larger. She produces relatively few eggs, is fertile for much less time than men, and bears the cost of pregnancy and childbirth. Her best strategy is to carry out behaviours that increase the survival chance of her children. Males have much lower parental investment, and can have many more children than women - they have a much larger fertility window and produce far more sperm than women produce eggs, and the only really limiting factor is access to willing females. This makes promiscuity a much more effective reproductive strategy.
Gross and Shine (1981) provide supporting evidence for Parental Investment Theory (PIT) by finding that in other species, the sex with parental certainty is most likely to be the primary caregiver. In species with internal fertilisation, such as humans, parental care is carried out by the females in 86% of cases. In species with external fertilisation, where the male has parental certainty rather than the female, parental care is carried out by the male in 70% of cases. The "deserter" parent doesn't have parental certainty, so they invest less parental care. However, the fact that there wasn't an 100% correlation suggests that factors other than fertilisation must affect parental investment.
Buss (1993) found further supporting evidence for PIT in a study that found that males show a greater stress response when imagining their partners being sexually unfaithful, whereas females show a greater stress response when imagining their partners being emotionally unfaithful. According to PIT, males will show more jealousy when their partners are sexually unfaithful as this decreases their parental certainty, and females will show more jealousy when their partners are emotionally unfaithful, as this puts them at risk of losing the resources their partner provides, and this study supports those hypotheses.
(Supporting evidence for differences in parental investment come from Clark and Hatfield's 1989 study into attitudes towards casual sex, where gender differences in attitude were explained by differences in optimal reproductive strategy and parental investment. Being unable to ensure parental certainty, men's optimal sexual strategy is promiscuity, and this was reflect in the study's results - when offered casual sex with a stranger, all women in the study declined, whereas 75% of men accepted - compared to only 69% agreeing to go back to the woman's house!)
Clark and Hatfield's study uses a potentially biased sample, approaching only university students in their research. Certain aspects of university culture make casual sex more acceptable and encouraged than in other sectors of society, so their results cannot necessarily apply to the general population.
The study above can be used as AO2 for both parental investment and reproductive behaviour questions - just make sure you specifically apply it to the question and make it clear how it links to either differences in behaviour or investment depending on the question.
A key issue with studies into promiscuity is that cultural factors could have affected the validity of the results. There is more of a social stigma to promiscuity in women than in men, so female participants in Clark and Hatfield's study could have been less likely to accept the offer of casual sex than men because of the social taboo, rather than because of evolutionary differences in parental investment.
Cultural bias is also an issue here when trying to apply results globally - the reported disparity in sexual strategies could be more a product of cultural norms than evolutionary differences in parental investment, and therefore would not apply cross-culturally. Casual sex and promiscuity is much more acceptable in some cultures than others - it is imposing an etic to generalise Clark and Hatfield's results from an American study to less tolerant countries like Saudi Arabia.
The concept of parental certainty affecting parental investment is supported by a study by Nettle (2007), finding maternal grandparents had more contact with children than paternal grandparents, and offered more financial support and care for their grandchildren. This is what evolutionary theory would suggest - as only maternal grandparents can be certain of their genetic link to the child. However, this study fails to account for social norms - it's a cultural expectation that maternal grandparents will have a closer relationship to their grandchildren than paternal ones will, and contribute more.
The evolutionary approach is overly deterministic, stating that we choose mates to reproduce, and that females select a mate for resource provision, whereas males want to spread their genes as widely as possible, investing less in a single child than females do, but having more children overall. There are obvious counter-examples where free will overrides the evolutionary past - couples where the man is younger than the woman, heterosexual couples without children, homosexual couples, stepparents who love and care for their stepchildren. It is too deterministic to suggest that human reproductive behaviour and parental investment is entirely defined by evolutionary adaptiveness, especially as social change leads to further emphasis on free will in mating and parenting.
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