Sex differences in brain and behavior of animals including humans result from an interaction between biological and environmental influences. This is also true for the differences between men and women concerning sexual orientation. Sexual differentiation is mediated by three groups of biological mechanisms: early actions of sex steroids, more direct actions of sex-specific genes not mediated by gonadal sex steroids and epigenetic mechanisms. Differential interactions with parents and conspecifics have additionally long-term influences on behavior. This presentation reviews available evidence indicating that these different mechanisms play a significant role in the control of sexual partner preference in animals and humans, in other words the homosexual versus heterosexual orientation. Clinical and epidemiological studies of phenotypically selected populations indicate that early actions of hormones and genetic factors clearly contribute to the determination of sexual orientation. The maternal embryonic environment also modifies the incidence of male homosexuality via immunological mechanisms. The relative contribution of each of these mechanisms remains however to be determined.

包括人类在内的动物大脑和行为的性别差异是生物和环境影响之间相互作用的结果。男性和女性在性取向方面的差异也是如此。性别分化由三组生物机制介导：性类固醇的早期作用、非性腺性类固醇介导的性别特异性基因的更直接作用和表观遗传机制。与父母和同族的不同互动也会对行为产生长期影响。本演讲回顾了现有的证据，表明这些不同的机制在控制动物和人类的性伴侣偏好（即同性恋与异性恋取向）方面发挥着重要作用。对表型选择人群的临床和流行病学研究表明，激素和遗传因素的早期作用显然有助于性取向的确定。母体胚胎环境还通过免疫机制改变男性同性恋的发生率。然而，这些机制的相对贡献仍有待确定。