Many unknowns.
Neurobiology of gender identity and sexual orientation
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CONCLUSIONS
The data summarised in the present review suggest that both gender identity and sexual orientation are significantly influenced by events occurring during the early developmental period when the brain is differentiating under the influence of gonadal steroid hormones, genes and maternal factors. However, our current understanding of these factors is far from complete and the results are not always consistent. Animal studies form both the theoretical underpinnings of the prenatal hormone hypothesis and provide causal evidence for the effect of prenatal hormones on sexual orientation as modelled by tests of sexual partner preferences, although they do not translate to gender identity.
Sexual differentiation of the genitals takes place before sexual differentiation of the brain, making it possible that they are not always congruent. Structural and functional differences of hypothalamic nuclei and other brain areas differ in relation to sexual identity and sexual orientation, indicating that these traits develop independently. This may be a result of differing hormone sensitivities and/or separate critical periods, although this remains to be explored. Most findings are consistent with a predisposing influence of hormones or genes, rather than a determining influence. For example, only some people exposed to atypical hormone environments prenatally show altered gender identity or sexual orientation, whereas many do not. Family and twin studies indicate that genes play a role, but no specific candidate genes have been identified. Evidence that relates to the number of older brothers implicates maternal immune responses as a contributing factor for male sexual orientation. All of these mechanisms rely on correlations and our current understanding suffers from many limitations in the data, such as a reliance on retrospective clinical studies of individuals with rare conditions, small study populations sizes, biases in recruiting subjects, too much reliance on studies of male homosexuals, and the assumption that sexuality is easily categorised and binary. Moreover, none of the biological factors identified so far can explain all of the variances in sexual identity or orientation, nor is it known whether or how these factors may interact. Despite these limitations, the existing empirical evidence makes it clear that there is a significant biological contribution to the development of an individual’s sexual identity and sexual orientation.
Neurobiology of gender identity and sexual orientation