Are all males equal? Anatomic and functional basis for sexual orientation in males.

In non-human primates anterior hypothalamic nuclei are closely involved in heterosexual activity in males. In humans, hypothalamic nuclei which correspond to these nuclei in non-human primates have been shown to have a neuronal density in homosexual men that is approximately half of that seen in heterosexual men. In addition, homosexual men exhibit a positive luteinizing hormone response to acutely administered estrogen that is intermediate between women (flat response) and heterosexual men (exaggerated response). Furthermore, on the basis of serum testosterone concentrations in similarly feminized transsexual males on estrogen/progestogen treatment 3 distinct groups can be identified. It is postulated that anatomic differences in the anterior hypothalamic nuclei that regulate sexual orientation in males may lead to alteration in the gonadotropin releasing hormone (GnRH) pulse/frequency leading to a more female-type pattern of gonadotropin secretion in homosexual males. Based on data in transsexual males this pattern may be more or less of the female-type in subsets of male homosexuals.