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

Although an increasing number of studies in mammals provide support to the Trivers-Willard prediction, evidence of this phenomenon in human remains controversial. 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 population 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, suggesting 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 investment in costly male offspring when resources are abundant. They also suggest that mechanisms that might have been adaptive under ancestral conditions can produce maladaptive population-level consequences in the modern world.