Prenatal Influences on Human Sexual Orientation: Expectations versus Data

Abstract

In non-human vertebrate species, sexual differentiation of the brain is primarily driven by androgens such as testosterone organizing the brains of males in a masculine fashion early in life, while the lower levels of androgen in developing females organizes their brains in a feminine fashion. These principles may be relevant to development of sexual orientation in humans, because retrospective markers of prenatal androgen exposure, namely digit ratios and otoacoustic emissions, indicate that lesbians, on average, were exposed to greater prenatal androgen than were straight women. Thus the even greater levels of prenatal androgen exposure experienced by fetal males may explain why the vast majority of them grow up to be attracted to women. However, the same markers indicate no differences between gay and straight men in terms of average prenatal androgen exposure, so the variance in orientation in men cannot be accounted for by variance in prenatal androgen exposure, but may be due to variance in response to prenatal androgens. These data contradict several popular notions about human sexual orientation. Sexual orientation in women is said to be fluid, sometimes implying that only social influences in adulthood are at work, yet the data indicate prenatal influences matter, too. Gay men are widely perceived as under-masculinized, yet the data indicate they are exposed to as much prenatal androgen as straight men. There is growing sentiment to reject “binary” conceptions of human sexual orientations, to emphasize instead a spectrum of orientations. Yet the data indicate that human sexual orientation is sufficiently polarized that groups of lesbians, on average, show evidence of greater prenatal androgen exposure than groups of straight women, while groups of gay men have, on average, a greater proportion of brothers among their older siblings than do straight men.

Human Sexual Orientation: The Importance of Evidentiary Convergence

“…News such as this is a constant reminder that it remains important to periodically restate the following: homosexuality is not a choice; rather, it is largely the result of biological factors acting mostly during prenatal life (Bailey et al, 2016; Ernulf, Innala, & Whitam, 1989) potentially interacting with various post-natal influences, both types of influences eventually varying between men and women (Diamond & Rosky, 2016). In this context, there were aspect of the Target Article by Breedlove (2017) that we found perplexing.…”

“…This marker has also been shown to be reliable in terms of the average differences between men and women and also between homosexual and heterosexual women. Multiple studies, including meta-analyses (Grimbos, Dawood, Burriss, Zucker, & Puts, 2010), have confirmed the statistically significant differences between average values in these two types of populations (for a detailed list of references, see Breedlove, 2017). …”

Neurobiology of gender identity and sexual orientation

“…The 2D:4D ratio is generally smaller in men than in women, although the validity of this measure as a marker influenced by only prenatal androgen exposure has been questioned . Nonetheless, numerous studies have reported that the 2D:4D ratio is also on average smaller in lesbians than in heterosexual women, a finding that has been extensively replicated and suggests the testosterone plays a role in female sexual orientation. Similar to OAEs, digit ratios do not appear to be feminised in homosexual men and, similar to AEPs, may even be hyper‐masculinised.…”

“…Similar to OAEs, digit ratios do not appear to be feminised in homosexual men and, similar to AEPs, may even be hyper‐masculinised. The lack of evidence for reduced androgen exposure in homosexual men (based on OAEs, AEPs and digit ratios) led Breedlove to speculate that there may be as yet undiscovered brain‐specific reductions in androgen responses in male foetuses that grow up to be homosexual. No variations in the human androgen receptor or the aromatase gene were found that relate to variations in sexual orientation .…”

Reconsidering Prenatal Hormonal Influences on Human Sexual Orientation: Lessons from Animal Research

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Relying primarily on indirect putative indices of fetal testosterone (T) exposure (2D:4D digit ratios and otoacoustic emissions), Breedlove (2017) summarizes literature supporting the conclusion that lesbian women, on average, are exposed to greaterprenatal androgen than are straight women. By contrast, Breedlove argues that homosexual orientation in men (based on these same putative retrospective markers of fetal androgen action) cannot be easily explained by a reduction in prenatal androgen.

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