Sex Differences in Normal Fetuses and Infants: A Commentary

Sex differences in infants warrant attention, not because they clarify the extent to which such differences reflect nature or nurture, but because studying them is likely to illuminate the origins of sex differences later in life and thereby yield manipulations that could influence the development of important competences. It is not yet clear how male and female infants come to differ. Testosterone is influential, but because of the complexity of the developmental systems in which it operates, its effects are not straightforward: Testosterone does different things in different contexts. Simple explanations invoking hormone exposure should not be expected to satisfactorily answer questions about the origins of sex differences, but standardizing protocols to allow meaningful meta-analyses would help bring coherence to the research literature in this domain.