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Brain Sex Differences
Your gender, whether you are a boy or girl, man or woman, is one of the most salient and enduring identifying features one possesses. The term gender is unique to humans as it incorporates both self and societal perceptions of one’s sex, which is either male or female. Thus, studies of animals address sex differences, whereas studies of humans may involve either or both gender and sex. One’s sex is determined on several levels that can be summarized as ‘the three Gs’ – genes, gonads, and genitalia (Joel, 2012).
In the overwhelming majority of cases, these three variables align such that an XX individual will develop ovaries and female genitalia whereas an XY individual will develop testis and male genitalia and this will in turn inform gender. Thus, sex and gender are internally consistent. But is this also true for the sex/gender of the brain? That boys and girls, men and women, behave differently is so self-evident as to be hardly worth stating. But this easy generality is in reality highly nuanced and complex in both its origins and manifestations. In what ways are men and women’s brains really different? And why are they different? Is it biology? Culture? Society? Experience? Or some combination thereof?
Only in animals can we hope to tease out truly biological sources of variation in male and female brains and the first experimental evidence of this comes from the publication of the now iconic paper of Phoenix, Goy, Gerall, and Young (1959). This study of guinea pigs established that the sensitivity of adult animals to either male (testosterone) or female (estradiol and progesterone) hormones and the induction of sexual behavior were dependent upon the hormonal milieu experienced early in life, with the authors asserting that the neural substrate controlling behavior had been ‘organized’ (Figure 1).
This was a heretical idea at the time but in hindsight is entirely consistent with other sensitive periods in brain development that alter adult neural function, as well as newly emerging ideas about early life programming that impacts all manners of adult responding including energy utilization, stress responding, and immune system activation (Bale et al., 2010). The authors made the even further heretical assertion that testosterone and its metabolites alter the structure or function of the neural correlates of sexual behavior. Today, we fully embrace the notion that steroids act on the brain to modify neural structure and function, regardless of whether one accepts this contributes to sex differences in behavior. In fact, the more we look, the more we find. But not all sex differences are made equal, and some may have evolved in order to appear only in response to challenge or to compensate for the costs of reproduction that differ in males and females (McCarthy et al., 2012 and McCarthy and Konkle, 2005).
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