Parental Investment

It is debated whether evolutionary explanations of parental investment accurately explain human behaviour.

Arguing for the importance of evolutionary factors, Trivers (1972) explains sexual selection in humans through the level of investment in child rearing made by men and women (Parental Investment Theory; PIT). As a woman has limited reproductive possibilities (one ovum per month), she uses inter-sexual selection to pass on her "selfish genes" (Dawkins) by selecting (inter-sexual selection) a male who can provide for (with financial status) and protect (muscular) her offspring until they reach maturity.  As such, she is also biologically motivated to invest more in her children (as she must care for the offspring during pregnancy and childhood). Conversely, as a man has numerous reproductive possibilities (millions of sperm a day) he can be less selective, ensuring that he passes on his selfish genes by competing (intra-sexual selection) to procreate with as many fertile partners as possible. As a result, he is biologically motivated to select signs of youth and fertility, and is less likely to invest in his children. Adding to PIT, Internal Fertilisation Theory (IFT) argues that a woman is biologically motivated to take care of a child because she can be sure that it shares her genes (as it is internally fertilised in her). However, her male partner cannot be sure that the child is his, making him less motivated to care for the child and more motivated to invest in further reproductive opportunities.

Convincing evidence to support the roles of evolutionary factors in parental investment comes from Pollet (2007), who found that over 30% of maternal grandparents had regular contact with their grandchild, where only 15% of paternal grandparents did the same. This is because the mother's parents can be sure that the child shares her genes and are biologically motivated to spend child with their kin.  

Further strong evidence to support the role of evolutionary factors comes from Singh, who showed that men select a partner with a hip to waist ratio of 0.7, suggesting child-bearing hips (a sign of fertility); and Gross , who showed that women are attracted to a shoulder to waist ratio of 0.85-0.9, suggesting broad shoulders (a sign of protection).  As these were shown cross-culturally, it adds strong research support for the role of evolution.  

Finally, further suggesting the importance of these evolutionary factors, Dunbar and Wainforth carried out a content-analysis of personal adverts to show that 42% of men sought a younger partner, who demonstrated characteristics of physical fitness (fertility), whereas the majority of women sought an older partner who shows characteristics associated with success (e.g. industriousness) , again as suggested by the theory.

However, undermining the role of evolutionary factors in parental investment, this theory can be criticised for being simplistic because it ignores other important influences. This is because the theory stresses the importance of heterosexual procreation and therefore ignores homosexuality; couples who decide not to have children or to adopt; and blended families (step children).

For the same reasons, it can be criticised for being constrained and alpha biased, as it ignores the idea of free will and unfairly exaggerates the differences between men and women. This is because it suggests that we are genetically programmed to select the exaggerated characteristics proposed by the theory, which is obviously not always the case (e.g. in Dunbar and Wainforth’s research, the majority of male participants actually sought a partner of the same age or older). 

Also, further undermining the role of evolutionary factors, parental investment theory can be criticised for being historically biased as it was developed in one time and unfairly applied to others. This is because it was created in 1972 and therefore does not reflect the cultural changes that have occurred since that time (e.g. female breadwinners and male househusbands).

Finally, it can also be criticised for being unfalsifiable as it cannot be tested using an independent or dependent variable, and speculative, as it makes assumptions that go beyond the findings of research. This is because it is impossible to test an evolutionary theory as the period of evolutionary adaptation occurred millions of years ago. 

Therefore, whilst evolutionary theories of parental investment are compelling, they can never wholly be accepted because they are unfalsifiable.