Parental Investment Theory

Biological Background

In many species, individuals compete with each other for limited resources such as food and mates. Parenting poses the additional problem of trading off investing in offspring versus investing in oneself or other mates. Creating and rearing offspring is often costly, and this is especially true for female mammals, which are biologically obligated to invest their energy in their offspring during gestation.

This investment often continues following the birth of their offspring to include breastfeeding, caretaking, securing food, and protection from predators. Male mammals are typically not obligated to invest in their offspring once they have contributed their sperm. In many mammalian species, male parental investment is not necessary for offspring survival (e.g., cattle, orangutans). Thus, there is a biologically based cost asymmetry for males and females in terms of parental investment in their offspring; the cost for females is nearly always far greater. Thus, the sex difference in paternal investment in offspring in many species is one of quantity versus quality: Males seek to maximize the number of offspring they can produce by seeking multiple mates, whereas females seek to maximize the survival and viability of the offspring they produce.

There is some evidence that offspring who receive some paternal investment are more likely to survive and reproduce than those who receive less or no paternal support. From an evolutionarily perspective, if offspring require investment to thrive and survive, and if paternal investment gives offspring a survival or reproductive advantage over and above maternal investment, then genes favoring paternal investment may be naturally selected for over evolutionary time. Thus, species that currently exhibit some forms of paternal investment, such as modern humans, owl monkeys, and siamangs (a lesser ape), may have faced selection pressures in the evolutionary past that favored some paternal investment over none (e.g., scarce resources). In humans, the protracted length of defenselessness in infancy and early childhood may have facilitated the natural selection of paternal investment over evolutionary time.

Parental Investment Theory seeks to explain the balancing act among the biological drives of the mother, the father, and their offspring. It is a balancing act because there are inherent cost-benefit tradeoffs. Investing in offspring often incurs a substantial cost on the investing parent. Energy that parents could use to promote their own health or to find better or additional mates instead gets channeled into promoting the health and development of their offspring. Although the offspring are the direct beneficiaries of parental investment, the parents may later reap the indirect benefits of having their genes passed ontothenextgenerationshouldtheir [Page 1195]offspring survive and reproduce. There is, however, an additional cost-benefit asymmetry: The cost of reproduction for males (e.g., contributing sperm) is far less than the cost of reproduction for females (e.g., months of pregnancy, breastfeeding, caretaking, and protection). Thus, in many mammalian species, males are more likely to spend their energy competing with other males to gain access to additional mates, whereas females are more likely to devote more of their energy to providing nurturance and protection for their offspring.

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