The theory of parental investment, developed by Robert Trivers in the 1970s, has become a fundamental concept in evolutionary biology and psychology for explaining sex differences across species. The theory states that the sex that invests more resources in raising offspring will become the ‘choosier’ sex in mate selection. In most species, including humans, females invest more heavily in bearing and raising children due to factors like internal gestation, breastfeeding etc. Thus, females are expected to be more selective in choosing mates. This theory provides a functionalist explanation for sex differences in traits related to mating strategies, aggression, mate selectivity and more. The theory has greatly influenced research on human and animal mating behaviors and sex differences in motivations and values. Here we review the basics, empirical support and implications of the theory of parental investment.
Parental investment theory proposes that sex differences arise from asymmetric parental investment
The theory of parental investment was formally proposed by Robert Trivers in 1972 to explain how sex differences in parental investment can shape the evolution of mating strategies and associated traits in males and females. The key premise is that the sex that invests more resources into producing and raising offspring will become the ‘choosier’ sex when it comes to mating. This is because higher investment makes fitness more sensitive to mate choice for that sex. The lower investing sex, on the other hand, can increase reproductive success by mating more indiscriminately due to their lower cost per offspring. In species where males invest relatively less than females in offspring, they are expected to compete more vigorously for access to mates, be more aggressive, and less discriminating of mates. The reverse is expected when females invest less.
Evidence across species supports predictions from parental investment theory
A wealth of empirical evidence from diverse species supports the central predictions from parental investment theory regarding mating behaviors and sex differences. For instance, in pipefish and seahorses where males care for the developing young, female competition and aggressiveness is higher than in related species with conventional sex roles. In species like dunnocks and jacanas where males solely provide parental care, females are highly competitive and promiscuous. In contrast, in species with female-only care like most mammals, male competition for mates is more intense. Sex differences in choosiness and promiscuity are also consistently associated with patterns of parental investment across diverse mating systems.
Parental investment patterns in humans align with evolutionary predictions
In humans, women bear a greater minimum obligatory parental investment due to 9 months of internal gestation during pregnancy, childbirth, and prolonged lactation after birth. Consistent with parental investment theory, men compete more vigorously for mates, are more sexually eager, prone to risk-taking and short-term mating, while women are choosier. Human mating involves a mix of long-term monogamy and some degree of polygyny, again consistent with sex differences in parental investment. However, social and cultural factors also interact with evolved predispositions to shape modern mating behaviors.
Parental investment theory explains evolution of sex differences beyond mating behaviors
The effects of asymmetric parental investment are not limited just to mating behaviors but extend to physical traits, attitudes, personality, sexuality and more. For instance, the greater male intrasexual competition is associated with greater male size and strength across mammals including humans. Sex differences in attitudes and values like men’s lower empathy or women’s stricter sexual morality can also be functionally explained from the lens of parental investment. However, as with mating strategies, evolved predispositions interact with social roles and norms to produce observed sex differences.
Critiques and limitations of the parental investment theory
While parental investment theory remains highly influential, it is not without limitations. Some critics argue it oversimplifies the diversity and flexibility of mating systems. The theory focuses specifically on variance in reproductive success arising from mating strategies, but other evolutionary forces also shape sex roles and differences. There are also alternative perspectives like social role theory which contend that sex differences arise primarily from division of labor and social status rather than evolved adaptations. However, the theory’s core premise that asymmetric investment in offspring selects for different mating strategies in males and females remains widely supported.
In summary, parental investment theory proposes that sex differences in reproductive strategies and associated traits arise fundamentally from asymmetric parental investment between the sexes. The sex investing more in offspring becomes choosier and competitors less strongly for mates. There is substantial evidence that patterns of parental care align closely with sex differences in mating behaviors across diverse species, including humans. Parental investment differences likely also contributed to evolution of sex differences extending beyond mating to factors like size, attitudes, personality and more. While critiqued for oversimplifying diversity in mating systems, the theory remains highly influential in explaining sex differences from an evolutionary perspective.