9 - Do Sex Steroid Hormones Contribute to Sexual Differentiation of the Human Brain?

Abstract

This chapter reviews sex steroid hormones and neural sex differences in humans, while focusing on the evidence, gaps in evidence, and methodologic challenges relevant to the question of whether sex steroids contribute to human neural sex differences. Sex differences are reported in a variety of neural regions, with many being homologous to sexually-differentiated and steroid-sensitive regions in experimental animals. Males tend to have larger brains and more white matter or cerebrospinal fluid, whereas females have proportionally more grey matter and, possibly, enhanced neuropil. High-resolution techniques suggest reliable sex differences for certain hypothalamic-related nuclei. Cortical volumes or the volumes adjusted for differences in brain size are reported to be greater for females in certain frontal and medial paralimbic areas and the superior temporal gyrus. In particular, females have increased dorsolateral prefrontal cortex, whereas males have greater age-related tissue loss there and potentially in other brain areas. Conversely, greater male volumes have been reported in frontomedial cortex and the inferior parietal lobule, although conflicting results have been reported for the parietal lobule. Evidence is accumulating that sex steroid hormones influence some of the gender differences in human behavior. Evidence from experimental animals suggests that sex steroid effects may depend upon the specific hormone of exposure, for example, natural estrogens vs. synthetic estrogens vs. testosterone vs. testosterone metabolites and the period of life, prenatal or early postnatal vs. adulthood.