Although the literature on language change has often replicated and discussed a pattern in which female speakers lead in changes that occur below the level of awareness, there is no consensus on why this pattern should arise. Interestingly, recent findings in endocrinology show that differences in prenatal testosterone exposure can impact learning patterns. In the light of these findings, we first present preliminary results consistent with the hypothesis that a biological factor, prenatal exposure to androgens, can have a small, continuous biasing effect on linguistic variation, namely the variable duration of pre-aspiration conditioned by voiceless obstruents in Tyneside English. Second, we propose an explanatory model in which the biological factor—prenatal testosterone exposure—creates subtle bias in how speakers learn linguistic variants and suggest that some reported sex effects are derivative. This model is compatible with the high tendency for females to lead in language change from below (Labov 1990: 206).