Parental Investment

Parental investment (PI), in evolutionary biology and evolutionary psychology, is any parental expenditure (time, energy etc.) that benefits one offspring at a cost to parents' ability to invest in other components of fitness (Clutton-Brock 1991: 9; Trivers 1972). Components of fitness (Beatty 1992) include the wellbeing of existing offspring, parents' future reproduction, and inclusive fitness through aid to kin (Hamilton, 1964).

It is held that parental investment starts from the point when the male and female copulate and the egg is fertilized. The minimal obligatory parental investment for a human male is to impregnate the woman and the time it takes to copulate. On the other hand, the minimal obligatory parental investment for a human female is her egg, nine months of pregnancy and delivery. In that case the female investment outweighs the male investment. The difference of minimal obligatory investment between males and females suggests that the amount of investment and effort put into mating and parenting will also differ. In theory, a man could impregnate any reproductive age woman who is fertile, leading to a large number of offspring from the male. In contrast, a woman can only have one offspring in nine months, limiting the amount of children she can have. This suggests that males should be more competitive between one another and women will be more ‘choosy’ because of the amount of investment, searching for the male with best fitness and good genes to pass onto her offspring (Trivers 1972). Such examples are relevant and seen in non-Western societies where men are allowed to have multiple wives, leading to a large number of offspring for example in Arabian families and some African tribes.

Parental investment theory accounts for many of the differences between males and females: these were evolved in order to survive and reproduce. The importance can be seen in modern humans. Human males spend more time caring for their offspring than other male mammals (Bjorklund&Shackelford 1999). This higher parental investment is the result of extended childhood of human offspring. Prolonged human childhood is required in order to develop the brain. Optimally, children learn how to survive, as well as learning about the society and its vices and virtues. However, this requires parental investment in the form of parents ‘leading the way’- teaching and protecting children. Young children left without care would simply die (due to malnutrition or victimized by predators). However, in some societies these problems still persist, for example in some countries of Africa where massive famine occurs, parents simply do not care enough about nourishing their children to procure resources to do so. In some cases societies have developed various means of caring for children when their parents do not. Males do spend time caring for their children but to a much smaller degree than mothers. This translates into a general observation that females’ parental investment is much greater than that of males, both before and after childbirth.