On the elusive nature of sex differences in cognition: hormonal influences contributing to within-sex variation.

We argue that within-sex variation resulting from the prenatal organizational and adult activational effects of gonadal steroid hormones has the potential to obscure between sex differences in cognitive performance and functional cerebral asymmetry. Two putative markers for prenatal testosterone, finger ridge count (FRC) asymmetry and the 2D:4D finger length ratio, have been linked to within-sex variation in cognitive performance. In particular, FRC allows the identification of men and women who show a reversal of the typical sex-related pattern of task performance. Three paradigms for the study of activational effects--seasonal, menstrual, and diurnal hormonal cycles--have evaluated changes in task performance and functional cerebral asymmetry. The performance of sex-dimorphic, but not sex-neutral, tasks changes with estrogen across the menstrual cycle and with testosterone across its seasonal and diurnal cycles. Functional cerebral asymmetry also changes systematically across both the menstrual cycle and the diurnal testosterone cycle in such a way that suggests left hemisphere performance increases as testosterone levels decline whereas right hemisphere performance increases as estrogen levels decline. In studies of sex differences, such correlates of within-sex hormone-related differences are rarely measured or controlled. Whatever the explanation for the associations of putative markers and hormone cycles with differences in cognitive abilities and cerebral asymmetry, it is clear that these relationships have the potential to contribute to the elusive nature of sex differences in cognition and functional brain organization.