Ray and colleagues1 present data on variation in birth sex ratios among Ontario newborns, showing a significant male-biased sex ratio at birth among multiparous Indian and South Korean mothers. The authors suggest that women from these countries may be using prenatal sex determination and selective termination. However, there are plausible, adaptive reasons for variations in both individual- and population-level sex ratios.
Although even sex ratios seem to be a natural consequence of chromosomal sex determination, such accounts fail to explain why natural selection favours such sex ratios. Fisher2 first argued that parents who overproduce the rarer sex will have greater evolutionary success. If such overproduction was transmitted genetically to offspring, then the rarer sex would become increasingly common over time and the advantage of producing the rarer sex would decrease, eventually disappearing when neither sex was rare (even sex ratio).
Fisher’s reasoning has been generalized to explain biased birth sex ratios, like those seen in humans. Besides differential, sex-specific mortality,3,4 biased sex ratios are an expected evolutionary response to inbreeding,4 competition or cooperation among relatives,4 and heritable, fitness-enhancing traits of parents.4 The latter explanation has been used to account for differential production of sons and daughters in humans.3,4 Furthermore, such adaptive responses can be achieved through a variety of physiological mechanisms, not simply feticide, and can result in sex-ratio bias at the population level5 similar to that observed by Ray and colleagues.
Given the multiplicity of factors that could contribute to adaptive variation in sex ratios,4 the results presented by Ray and colleagues seem less surprising, and possibly less troubling. From an evolutionary perspective, interpreting these results with caution — especially when they might have significant social and public-policy implications — seems wise.