Boy or girl: gender preferences from a Darwinian point of view.

This article reviews evolutionary biological studies of sex-biased post-natal parental investment that may be relevant to the issue of preconception gender selection. The focus is on tests of the Trivers-Willard hypothesis, which predicts that natural selection has favoured parents that bias investment in favour of the sex with the best reproductive prospects. Because resource abundance and scarcity often have greater effects on male than on female reproductive success, the Trivers-Willard model predicts that natural selection will most often favour parents who favour males when conditions are good and females when conditions are poor. Empirical tests of this hypothesis are mixed in terms of the appropriateness of their methods and their relevance to the model. Tests with more appropriate measures of such key variables as parental investment tend more often to provide support for the hypothesis. The implications of these findings for the issue of preconception gender selection are briefly discussed.