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.

The androgen receptor gene, which dictates how the body responds to hormones like testosterone and DHT, is located on the X chromosome. Since men (XY) inherit their X chromosome from their mother, their genetic predisposition for androgen sensitivity is maternally inherited.

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.

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.

Scientific evidence suggests that a mother's feelings toward her fetus can imprint a core sense of self before birth. This is demonstrated by a case where a newborn rejected its mother's milk because the mother secretly had not wanted the child.

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.

Contrary to popular belief, women's bodies contain substantially more total testosterone than estradiol. This misunderstanding arises because tests use different units (nanograms vs. picograms) or focus only on "free" testosterone, understating its overall prevalence and importance.

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.

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.