Sexual strategies theory: historical origins and current status - The Use of Theory in Research and Scholarship on Sexuality

Journal of Sex Research, Feb, 1998 by David M. Buss

Charles Darwin's great discovery was that recurrent differential reproduction caused by differences in design attributes--natural selection--is the key to evolutionary change over time (Darwin, 1859). Because reproduction is central to the evolutionary process, domains closely linked with reproduction should be focal targets of selection pressures and, hence, loci for evolved mechanisms or adaptations. No domain is closer to reproduction than sexuality. If the process of selection has not affected the evolution of human sexuality, then it is unlikely to have affected domains less directly linked with reproduction.

Darwin adopted Spencer's phrase "survival of the fittest" to summarize the process of natural selection, but this choice was unfortunate. Survival is certainly critical. Many adaptations of organisms exist because they successfully overcame the forces that impeded survival, the "hostile forces of nature," such as parasites, diseases, food shortages, predators, and extremes of climate and weather. Our fears of snakes, spiders, heights, darkness, and strangers, for example, are likely the psychological remnants of our psychology of survival, sculpted in an environment long gone.

Many people who lack evolutionary expertise, however, equate evolution by natural selection with survival selection. This is a mistake. Differential reproductive success linked with differences in heritable design features, not differential survival success, is the core of the process of natural selection. Survival only becomes important to the extent that it is tributary to reproduction.

Darwin had a wonderful habit that serves as an exemplar to modern scientists. He kept a special notebook to write down observations that seemed to falsify his theory. Darwin did this because he realized that he had a tendency to forget these anomalous observations, more than a century ago presaging psychological research on cognitive biases such as the tendency of people to search selectively for evidence that confirms held hypotheses (Johnson-Laird, 1983).

In this notebook, Darwin wrote several observations that seemed puzzling based on his theory of natural selection. He noticed the brilliant plumage of certain bird species and wondered how it could possibly have evolved. Such plumage is energetically costly and renders the birds more vulnerable to predation. This seemed to contradict "survival selection," and indeed it was difficult to imagine how these cumbersome features could possibly aid reproduction.

Next he noticed that the two sexes were sometimes quite different in structure. Often the males were larger than the females, sometimes exceeding their weight by two or four times. Furthermore, even when the sexes were roughly the same size, it was not uncommon that the females were drab and the males displayed gaudy features--luminescent plumage, unwieldy antlers, and other strange features that appeared to have nothing at all to do with survival. Because the sexes faced similar survival problems, how could these sex differences evolve? Why did the males tend to possess the strange and gaudy features, whereas the females were often drab by comparison?

In response to apparent anomalies of this sort, Darwin (1871) fashioned what he believed to be a second evolutionary process, which he called sexual selection. According to sexual selection theory, characteristics that give organisms an advantage in the competition for mates, as contrasted with enhanced survival, can evolve. Sexual selection can operate through two processes. The first is intrasexual or same-sex competition. If members of one sex compete with one another, and the victors of these competitions gain preferential sexual access to mates, then whatever qualities lead to success in same-sex competitions will be selected and can evolve over time. The large antlers of stags represent a prototypical image of this sort of intrasexual selection, but the logic extends to all qualities that might give an advantage in same-sex competition. These might include athletic ability, piloerection (hair standing on end to scare away a competitor), social skills to enlist allies, or even a biting sense of humor that deters a rival. The key point is that whatever qualities lead to success in same-sex competitions can evolve because of the reproductive advantage that accrues to the victor through increasing sexual access.

The second component of sexual selection involves mate choice. If members of one sex display a consensus about the qualities that are desired in mates, then those who possess the desired qualities have a preferential mating advantage. Those lacking the desired qualities get shunned and selectively excluded. Because the descendants of this process are more likely to carry both the preferences and the characteristics preferred, the two may co-evolve over time (Darwin, 1871; see also Fisher, 1930/1958). Sexual selection through mate choice ultimately reduces to mate competition, because those possessing desired features have a competitive mating advantage over those of the same sex who lack the desired features.