Human preferences for sexually dimorphic faces may be evolutionarily novel.

A large literature proposes that preferences for exaggerated sex typicality in human faces (masculinity/femininity) reflect a long evolutionary history of sexual and social selection. This proposal implies that dimorphism was important to judgments of attractiveness and personality in ancestral environments. It is difficult to evaluate, however, because most available data come from large-scale, industrialized, urban populations. Here, we report the results for 12 populations with very diverse levels of economic development. Surprisingly, preferences for exaggerated sex-specific traits are only found in the novel, highly developed environments. Similarly, perceptions that masculine males look aggressive increase strongly with development and, specifically, urbanization. These data challenge the hypothesis that facial dimorphism was an important ancestral signal of heritable mate value. One possibility is that highly developed environments provide novel opportunities to discern relationships between facial traits and behavior by exposing individuals to large numbers of unfamiliar faces, revealing patterns too subtle to detect with smaller samples.

Does polygyny reduce fertility?

Polygyny can increase, decrease, or have no effect on fertility. Understanding how this can occur requires consideration of both the proximate determinants of fertility and the ultimate effects of polygyny as a female reproductive strategy. Several factors reduced the fertility of polygynous women in 19th century Utah, including marrying at an older age, marrying older men, and conflict between co-wives. Sterility did not explain the reduced number of children in polygynous women, nor is there evidence of a "dilution effect" from sharing a husband. If women could anticipate a reduction in their own fertility, why would they still choose polygyny? Evidence suggests that they chose it because the children of polygynous men had increased fertility, high enough to offset the low fertility of polygynous women themselves.