Gender-specific selection in utero among contemporary human birth cohorts.

Much literature argues that natural selection has conserved mechanisms by which stressed females cull frail males in utero. This argument implies that males from low sex ratio birth cohorts should, on average, live longer than those from high sex ratio cohorts. Research reports such associations but these tests use completed lifespan as the outcome and, therefore, must end with cohorts born in 1913 because too many males survive from more contemporary cohorts to determine average lifespan. The empirical literature does not, therefore, address whether selection affects male mortality in contemporary cohorts. We apply time-series methods to monthly cohorts born in California between 1989 and 2003 to measure the association between the ratio of male to female live births and infant mortality, controlling for all forms of autocorrelation that induce spurious correlations. Consistent with theories of selection in utero, we find a positive correlation between cohort sex ratio and male infant mortality. The results suggest that natural selection conserved the stress mechanism in females to end the gestation of relatively less fit males and that this mechanism manifests itself in contemporary human societies.