The relationship between sexual selection and human reproductive behaviour

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Differences in male and female human reproductive behaviour have evolved as a result of different evolutionary selective pressures between the genders. In sexual selection, traits are sought in the opposite sex that are indicative of reproductive success and genetic fitness, and desirable traits differ between genders due to human anisogamy - differences in male and female gametes, and in the degree of parental certainty each gender has.


Males produce many, highly mobile sperm, and can do this fairly regularly over a long period of time - having many hypothetical opportunities for reproduction. Females, however, produce small numbers of larger eggs over a shorter period of time - and have the additional cost of bearing and nurturing children. This gives them far fewer opportunities for reproduction than males, as well as a shorter fertility window. A female's best strategy is to be choosy - selecting the males to mate with who display both genetic fitness, and high levels of status and resources, encouraging males to compete with each other in courtship rituals. A male's best strategy is promiscuity - reproducing with as many fertile females as possible, and only form close bonds with a select few females if bonding reduces the chances of being cuckolded.


Indicators of fertility in women include a youthful appearance, a symmetrical face, and a waist: hip ratio of 0.7, producing the typical "hourglass figure", a preference that exists cross-culturally despite cultural preferences for curvier or slimmer figures. Desirable traits in males include wealth and status that suggest ability to provide for the female and their child, as well as indicators of fertility and genetic fitness such as a square jaw, body symmetry, above-average height and small eyes.


Dunbar and Waynforth (1995) found evidence that supports the predictions of evolutionary theory by showing the same gender differences in sexual selection that evolutionary theory hypothesises. From cross-cultural data collected from across 33 countries, males were found to value physical attractiveness and youth, supporting the idea that males seek fertility in females. Females were found to value financial capacity, ambition and industriousness in males - supporting the idea that females seek indicators of resource richness and the ability to provide for them and their children.


Although a common criticism of evolutionary theory is that predictions are impossible to directly test through experimental procedure, and therefore unfalsifiable and unscientific, Dunbar and Waynforth's study worked around this issue by devising a different methodology to test the theory. Rather than using an experimental design, they made predictions based on evolutionary theory's principles of sexual selection, and found evidence that their predictions were correct, treating evolutionary theory more scientifically than is possible through experiment alone.


Evidence that gender differences in human sexual selection affect reproductive behaviour comes from Clark and Hatfield's 1989 study into gender attitudes towards casual sex in university students. When approached with an offer of sex, all female "participants" declined, while 75% of male participants accepted. This supports the concept of different adaptive sexual strategies between genders - with a much more limited reproductive capacity than men, choosiness is more likely to be a beneficial strategy for women, whereas men's reproductive capacities are less limited by biological constraints, so promiscuity is a much more adaptive strategy. This link between adaptive sexual strategies and reproductive behaviour is evidenced by the gender differences in willingness to have casual sex.


However, Clark and Hatfield's sample is potentially biased, approaching only university students, and therefore not representative of the general population. Certain aspects of university culture make casual sex more acceptable and encouraged than in other sectors of society, so their results cannot necessarily be generalised, as acceptance rates may have been different.


A key issue with studies into promiscuity is that cultural factors could have affected the validity of the results. There is more of a social stigma to promiscuity in women than in men, so female participants in Clark and Hatfield's study could have been less likely to accept the offer of casual sex than men because of the social taboo, rather than because of evolutionary gender differences in reproductive behaviour.

Cultural bias is also an issue here when trying to apply results globally - the reported disparity in sexual strategies could be more a product of cultural norms than evolutionary gender differences in reproductive behaviour, and therefore would not apply cross-culturally. Casual sex and promiscuity is much more acceptable in some cultures than others - it is imposing an etic to generalise Clark and Hatfield's results from an American study to less tolerant countries like Saudi Arabia, so conclusions cannot necessarily be generalised.


Further evidence for gender differences in human sexual selection affecting reproductive behaviour comes from Buss and Schmidt's 1993 study into each gender's desired number of sexual partners. Over the course of the next two years, men chose an average of 8 partners compared to women's 1, and over the course of a lifetime, men chose an average of 18 partners compared to women's 4 or 5. Men seeking more partners is predicted by evolutionary theory - due to their higher reproductive capacity, men can afford to be less rigorous with their sexual selection criteria than women, and favour a reproductive strategy of promiscuity. With more of a reproductive limit, evolutionary theory predicts that women will be more selective in their choice of sexual partners, applying a reproductive strategy of choosiness. Both these predictions are supported by the study.


The evolutionary approach is overly deterministic, stating that we choose mates to reproduce, and that females select a mate for resource provision, whereas males want to spread their genes as widely as possible, investing less in a single child than females do, but having more children overall. There are obvious counter-examples where free will overrides the evolutionary past - couples where the man is younger than the woman, heterosexual couples without children, homosexual couples, stepparents who love and care for their stepchildren. It is too deterministic to suggest that human reproductive behaviour and sexual selection is entirely defined by evolutionary adaptiveness, especially as social change leads to further emphasis on free will in reproductive behaviour.


Furthermore, evolutionary theory's approach to sexual selection and reproductive behaviour is too reductionist, removing human relationships from their social and cultural context to focus only on biological metrics of fertility and genetic fitness. Many factors other than physical appearance and resource acquisition play a role in sexual selection, such as the social demographic variables, similarities of attitudes and beliefs, and the complementarity of emotional needs proposed by Kerkhoff and Davis' filter theory. To reduce human reproductive behaviour down to basic evolutionary traits is an oversimplification of an incredibly complex behaviour system.