While animal sexual behavior is often a series of fixed motor patterns, human sexuality is overwhelmingly characterized by *who* the partner is. This intense focus on partner gender, rather than the act itself, is a key distinction of our species.
Hormones shape brain circuits not just for attraction but also for aversion. This is seen in some male sheep ("gay rams") that consistently refuse to mount females, suggesting a powerful aversive component to their sexual preference, rather than just a lack of attraction.
Contrary to the belief that deep brain structures are fixed after development, the hypothalamus exhibits plasticity in adulthood. Nuclei can change size within weeks in response to hormonal changes, demonstrating that the brain remains dynamic and complicating interpretations of correlational brain studies.
The largest sex differences in human behavior are in sexual attitudes (e.g., interest in casual sex, multiple partners). In these domains, gay men's preferences are typically masculine and align with those of straight men, challenging stereotypes that they are behaviorally "feminized" or under-androgenized.
The 2D:4D finger length ratio is a marker for prenatal testosterone exposure. On average, lesbians exhibit a more masculine ratio (shorter index finger relative to ring finger) than straight women, suggesting a biological influence on sexual orientation established in the womb.
Congenital Adrenal Hyperplasia (CAH) exposes XX fetuses to high levels of androgens. As adults, these women are statistically more likely to have a same-sex orientation than the general population, providing a clear "natural experiment" linking prenatal hormones to human sexual preference.
Data and observation suggest women's sexual orientation is more fluid throughout their lives. Women are more likely than men to identify as straight for a period and later form same-sex relationships, indicating a higher degree of plasticity in female sexual preference.
Each male pregnancy can trigger a maternal immune response to male-specific proteins. These antibodies may cross the placenta in subsequent male pregnancies, altering brain development and increasing the probability of homosexuality. This is known as the fraternal birth order effect.
Otoacoustic emissions, tiny sounds produced by the inner ear, show a sex difference at birth. Studies found lesbians have fewer emissions than straight women, a pattern closer to males. This suggests prenatal testosterone, which influences these emissions, also plays a role in shaping sexual orientation.
Even large, statistically significant differences between groups, like height between men and women, have high error rates when predicting an individual's classification. For smaller differences, like finger-length ratios and sexual orientation, the predictive value for a single person is practically zero.
Unlike other primates, the human brain continues its rapid, fetal-like growth trajectory for years after birth. This protracted development period makes children uniquely receptive to intense social learning and environmental influences, effectively functioning as "external fetuses."
Studies on 2D:4D finger ratios, a proxy for prenatal androgen exposure, found no average difference between gay and straight men. This challenges the "under-androgenized" stereotype and suggests orientation differences may stem from the brain's response to testosterone, not the hormone level itself.