The evolutionary explanations of parental investment all believe that parents invest in their offspring in various ways, such as giving food and the energy put into rearing the child, and risks taken to protect them. This investment is defined as ‘any investment made by a parent in one of his or her offspring that increases the chance that the offspring will survive at the expense of the parent’s ability to invest in any other offspring (alive or yet to be born)’ by Trivers, 1972. The amount of parental investment however, differs between males and females.
Trivers, in 1972, came up with the parental investment theory. Central to this is the fact that men and women do not usually invest the same amount in their offspring. Women have to invest more to start with, as women have far less eggs, and these are harder to produce, than men do sperm. As well as this, females are limited to how many offspring they can have, whilst males can produce a virtually unlimited number. For this reason, females are typically concerned with the quality of a male and the resources they can supply, whilst males are more concerned with the quantities of females he can impregnate.
After birth, human women have babies that are far more immature that other species, due to evolution of scull size, so have to spend longer rearing their children, such as breastfeeding. There are two consequences of this maternal investment- females wish for male providers because of the dependency that their children have on them, and because of the effort in rearing children, women do not want their efforts wasted on bad quality offspring. For example, to prevent this they may marry a man with plenty of resources, but have an affair with a more attractive man, therefore cuckoldering their mate.
Human males, on the other hand, can choose not to invest much in their children. Conception takes very little, as they have an infinite amount of sperm and do not have to go through pregnancy. Following the birth, they have the choice to invest nothing at all. However, when they do choose to, they must protect themselves from cuckoldry, as have a larger concern for fidelity of their partner than women d (Miller, 1998). Whilst a man risked being cuckolded if his mate was unfaithful, women risked losing resources. It is thought that as an answer to these problems, sexual jealousy evolved (Buss, 1995). Supporting this is Buss et al. who found in 1992 that US male students showed more concerns about sexual infidelity but female students more worries over emotional infidelity, this was supported also by galvanic skin responses.
Joint parental investment is desirable however, as it means men can increase the success of childrearing through the resources they provide, (Dunbar, 1995b) causing males often to invest individually in their offspring. This shows why attractiveness of females is so important to male humans in comparison with males of other species.
Despite this theory explaining why it is shown that usually women select the mate, and males compete to be it, this is not always the case. For example, after WW2, when there were fewer males left than females, there was a baby boom, leading to the 1960s having more marriageable women than men available, which may explain the change in sexual morals (greater female competition meant a society more dominated by male mating strategies) or could have been due to the introduction of the Pill. This is supported by a study by Hill and Hurtado, (1996) who found when they studied two Indian cultures in South America, that although they both lived in very similar environments, the Hiwi had more men so had stable marital life, whereas in the Ache, there were more women and extramarital affairs were common, showing that sexual morality decreases when the ratio of women is higher to that of men.
This theory is credible, because it allows a way of explaining the link between sexual selection, mating behaviour and parental investment (Buss, 1998), and shows that when mating, men gain from polygyny and women monogamy. Though it has been shown that before westernisation, approximately 80% of human societies were polygamous, suggesting that sometimes, e.g. at times of limited resources, males and females both gain from polygyny regarding reproductive success. However, contradicting the positive link between resources and reproductive success is that in 21st century western society, reproduction is lowest among wealthier people (those who have most resources). This can be explained through the use of contraception and enforced monogamy in the socialisation of wealthier people (Perusse, 1993).
The offspring have been shown also to have an influence over the amount of parental investment. Trivers argues that because resource allocations to maximise parental fitness are not the same as those to maximise offspring fitness, that parents and offspring will disagree on when the child should be weaned, and parents will tend to punish conflict between siblings and reward cooperation, encouraging children to value siblings more than they are naturally inclined to. In the majority of species, offspring wish to gain more than their siblings do in terms of resources, so as to maximise their own fitness, causing sibling rivalry.
To reduce sibling rivalry, Lalumiere et al. (1996) suggested that siblings should have their individual strengths maximised and by encouraged along their own path, reducing sibling competition and discouraging attraction to the same mates. It is thought this may be what causes siblings to turn out differently, but Harris (1999) argues that it is because parental care does not have as great an effect as peer socialization.
Parent-offspring conflict starts from conception, when the foetus causes high blood pressure in the mother, by secreting hormones when it wants more nutrition. This causes more nutritious blood to travel to the foetus, benefiting it at the expense of the women carry it (Haig, 1998). It has been found though, that those mothers who have higher blood pressure through their pregnancy are less likely to suffer from spontaneous abortion (Haig, 1993) and tend to have larger babies (Xiong et al. 1993) suggesting it is an adaptive strategy to produce healthier babies. After birth, the conflict continues.
Natural selection causes parents to maximise fitness by investing in their current offspring, decreasing this investment in favour of younger offspring as they grow up. Resources are given more to younger children as these have a higher chance of dying before they reach adulthood than the older children do. This causes conflict, as the children want the parent’s main focus to remain on them for as long as possible. In comparison, it has been suggested by Salmon and Daly (1998) that younger children do not bother to compete with siblings for parental attention, instead acting cooperatively, so as to form alliances with non-kin. A contrasting non-evolutionary theory argues that younger children tend to be more co-operative because they learn to negotiate through coping with older siblings (Shaffer, 1993).