Discuss sex differences in parental investment. (8 marks + 16 marks)

For this question you need to consider how the investment of males and females differs and the consequences of this.


Human females make a greater biological investment than males because they produce far fewer egg cells than males produces sperm, and egg cells are far more costly to produce. Another reason why females have a greater initial investment is that females can only have a limited  number of offspring, whereas a male can potentially have a much larger number of children. As a result of this inequity, females are choosier in who they mate with.

As well as making a larger prenatal investment, human mothers must also make a larger postnatal investment. Childbirth in humans occurs relatively early in the fetus' development due to the large size of the human skull, meaning that human infants are born more immature than most other animals. Human females are consequently burdened by the extended period of childcare that results from this period of prolonged immaturity, which includes a longer period of breastfeeding. This is another reason why the investment of human females is particularly high.

The greater investment of females may also be explained in terms of parental certainty. Because fertilisation in humans is internal, the female is always certain that she is the mother of her child. The male, on the other hand, must always have some degree of parental uncertainty. Males are under pressure to protect themselves from investing resources in a child who is not genetically their own, and thus may be more reluctant to invest heavily in the child.

The possibility of sexual infidelity also posed different adaptive problems for males and females. A man whose mate was unfaithful risked investing in offspring who were not his own, while a women with an unfaithful mate risked the diversion of resources away from her & her children. Sexual jealousy, therefore, may have evolved as a solution to these problems. 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).

Evidence that parental investment by males is an evolutionary behaviour comes from comparative studies. Such research has found that in both chimpanzees & bonobos (the two most closely related species to humans), males show little or no parental investment. This suggests that the emergence of male parental investment is either an evolutionary change through our more recent ancestors or the contribution of evolutionary learning.

However, some psychologists view the evolutionary perspective of parental investment as limited and suggest that various personal & social factors determine men’s parental behaviour. These factors include the quality of the relationships with the mother and the personality characteristics of the father. Evidence for this is that childhood experiences such as divorce tend to correlate with the degrees to which men invest in the care of their children.

A consequence of the sex differences in parental investment is that some women may attempt to offset their greater parental investment by cuckolding their partners. The benefits which women could obtain from this behaviour include additional social or financial support from another male or higher-quality genes for a child. However, risks of this behave could include abandonment or the use of retention strategies (e.g. violence) by the female’s current partner.

Because men are liable to be unsure of their child’s paternity, they rely on the child’s resemblance to themselves. Daly & Wilson recorded conversations in a maternity ward and found that mothers often tried to strengthen the bond between father and son by commenting on their physical similarities, suggesting that an increased confidence in their child’s paternity can increase their investment in that child. However, this was a naturalistic observation, so there would have been many extraneous variables which would have compromised the study’s internal validity.

This theory of parental investment would predict that paternal investment would by greater if fathers know that the child is biologically theirs as they would be reluctant to spend time and resources bringing up someone else’s child. However, Anderson found that men invested no more resources in their biological children than in their step-children, challenging this assumption.

There is other research evidence to suggest that biological parents are more invested in their children than step-parents. Analyses of murders in the US found that children under the age of 2 are 60 times more likely to be killed by a step-parent than a biological parent. This supports the evolutionary theory of parental investment, as biological parents are less likely to kill children who contain their genes.

In line with the predictions of this theory of parental investment, Buss et al. found that male US students showed more distress when given a scenario of sexual infidelity while female students showed more distress for emotional infidelity. However, Harris found that men tended to respond with more arousal to any sexual imagery, challenging this view that sex differences in jealousy are an adaptive response in males & females.

A physiological approach can be used to explain the differential costs of parental investment among males & females. A study by Geher et al. found that males showed greater arousal of the autonomic nervous system than females when presented with examples that show the cost of parenting. Consistent with this theory of parental investment, males appear biologically less prepared than females to deal with the issues associated with parenting.

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