Do Mothers from Rich and Well-Nourished Countries Bear More Sons?

Abstract

Although an increasing number of studies in mammals provide support to the Trivers-Willard prediction, evidence of this phenomenon in human remains controver- sial. Here, assuming that contemporary humans respond in an ancestral manner to recent improvements of lifestyle, we explored the hypothesis of a facultative adjustment of sex ratio in relation to resource availability from more than 120 countries worldwide. Although maladaptive at the popula- tion level, we found that sex ratio at birth is significantly more male-biased in nations that are rich, well nourished, and with low fertility. The tendency was weak however, sug- gesting that the magnitude of this effect is small and/or that other processes act to maintain sex ratio equilibrium. These results provide support to the hypothesis predicting invest- ment in costly male offspring when resources are abundant. They also suggest that mechanisms that might have been adaptive under ancestral conditions can produce maladap- tive population-level consequences in the modern world.