Evolution designed an economical system where a single, subconscious "kinship estimate" for each person dictates both altruism towards them and sexual aversion. It's one calculation for two different social behaviors, determining how close your heart should be and how far your genitals should be.

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 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.

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.

Contrary to being an evolutionary 'paradox,' research suggests same-sex sexual behavior in animals is a functional adaptation for survival. Its prevalence increases in species facing greater external threats, such as extreme climates or high numbers of predators. This indicates the behavior serves to strengthen social cohesion and cooperation, which are critical for group survival under stressful conditions.

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.

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.

Research shows that individuals who are more easily disgusted in general also tend to exhibit more homophobic attitudes. This link is likely because sexuality involves bodies and fluids, potent disgust triggers, making it easy to elicit an aversive emotional response towards non-normative sexual acts.