Current evidence indicates that sexual differentiation of the human brain occurs during fetal and neonatal development and programs our gender identity-our feeling of being male or female and our sexual orientation as hetero-, homo-, or bisexual. This sexual differentiation process is accompanied by many structural and functional brain differences among these groups (1). In previous studies (2, 3), the Savic laboratory detected a sex-differentiated activation of the anterior hypothalamus in heterosexual men (HeM) and heterosexual women (HeW) and a sex-atypical, almost reversed, pattern of activation in homosexual men (HoM) and homosexual women (HoW). The hypothalamus (Fig. 1) is a small brain area located under the anterior commissure that is involved in many different functions, including reproduction. These observations raised several questions, one of which was whether the sexual dimorphisms described could be sex-atypical in homosexual subjects even with respect to factors not directly associated with reproduction. In a recent issue of PNAS, Savic and Lindstroem (4) reported that hemispheric ratios, as well as patterns of amygdala connectivity, were sex-atypical in homosexual individuals, with HoM exhibiting more female patterns than HeM and HoW showing more male-like features than HeW.
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