Sexual orientation is shaped by a combination of biological factors, not a single cause or a conscious choice. Decades of research point to genetics, prenatal hormone exposure, immune responses during pregnancy, and epigenetic patterns all playing a role. No one factor determines whether someone is gay, but together these influences shape the development of sexual attraction long before a person is aware of it.
About 5.5% of U.S. adults identify as LGBT, according to data from the Williams Institute at UCLA. That figure is higher among younger adults: roughly 15% of people ages 18 to 24 identify as LGBT, compared to less than 3% of those over 50. Whether this reflects greater openness among younger generations, broader definitions of identity, or both is still debated.
Genetics Play a Partial Role
Sexual orientation runs in families, and twin studies consistently show it has a heritable component. In studies of identical twins (who share the same DNA), if one twin is gay, the other is more likely to be gay than in the general population. But it’s far from guaranteed, which means genes contribute without being the whole story.
Large-scale genome studies have identified several regions of DNA associated with same-sex behavior, but each one has a tiny individual effect. There is no single “gay gene.” Instead, many genetic variants likely interact with each other and with the environment during development. Researchers at UCLA studied 47 pairs of identical male twins and found that a machine learning algorithm could predict sexual orientation about 70% of the time based on epigenetic patterns at just nine locations in the genome. Epigenetics refers to chemical modifications that change how active a gene is without altering the DNA sequence itself. In this case, a process called methylation, which dials down gene activity, differed between gay and straight identical twins who share the exact same genetic code.
Hormones in the Womb
One of the most studied biological influences is prenatal hormone exposure, particularly testosterone. During fetal development, sex hormones don’t just shape the body. They also organize the brain in ways that influence behavior later in life, including who a person is attracted to. This is sometimes called the organizational hypothesis: hormones set up the brain’s wiring early on, and puberty later activates those structures.
Researchers use finger length ratios (specifically, the ratio between the index finger and ring finger) as a rough indicator of how much testosterone a fetus was exposed to. Higher prenatal testosterone tends to produce a longer ring finger relative to the index finger. On average, lesbian women show more masculine finger ratios than straight women, suggesting greater testosterone exposure before birth. Among lesbian women who identify as more masculine-presenting, this pattern is even more pronounced.
The relationship isn’t straightforward, though. Evidence suggests it depends on the timing of hormone fluctuations, individual differences in how the body responds to hormones, and possible threshold effects where very high androgen levels can actually reverse the expected pattern. This complexity helps explain why prenatal hormones are one piece of the puzzle rather than a simple on-off switch.
The Older Brother Effect
One of the most replicated findings in sexual orientation research is the fraternal birth order effect: each older biological brother a man has increases his likelihood of being gay by a measurable amount. This doesn’t apply to older sisters, adopted brothers, or stepbrothers. It’s specific to biological brothers who shared the same womb.
The leading explanation involves the mother’s immune system. During pregnancy with a male fetus, the mother is exposed to proteins produced only by male cells. One protein in particular, called NLGN4Y, plays a role in how brain cells form connections. With each successive male pregnancy, some mothers build up antibodies against this protein. Those antibodies can cross the placenta and reach the developing brain of the next male fetus, potentially influencing how it develops.
Research published in the Proceedings of the National Academy of Sciences confirmed this biochemically. Mothers of gay sons, especially those with older brothers, had significantly higher levels of anti-NLGN4Y antibodies in their blood than mothers of straight sons. The pattern followed a clear gradient: women with no sons had the lowest levels, followed by mothers of straight sons, then mothers of gay sons without older brothers, and finally mothers of gay sons with older brothers at the highest levels.
Brain Structure Differences
Some research has found structural differences in the brains of gay and straight individuals, though this area remains less definitive. A small cluster of cells in the hypothalamus, a brain region involved in basic drives including sexual behavior, has been reported to differ by sexual orientation in men. In heterosexual men, this cell cluster (called INAH3) occupies a larger volume and contains more neurons than in women. In gay men, it tends to be smaller, closer in size to what’s typically seen in women.
These findings come from postmortem brain studies with limited sample sizes, so they should be interpreted cautiously. It’s also unclear whether these differences are a cause of sexual orientation, a result of it, or simply a parallel feature shaped by the same underlying developmental processes.
Why Evolution Didn’t Select It Out
A common question is why genes associated with same-sex attraction would persist across generations if gay individuals are less likely to have biological children. Several evolutionary theories address this. The kin selection hypothesis, proposed by E.O. Wilson, suggests that gay individuals may boost the survival of their family’s genes by investing heavily in nieces and nephews. By helping close relatives thrive, they indirectly pass on shared genetic material even without having children of their own.
Another explanation is that the same genetic variants associated with same-sex attraction in one sex may increase reproductive success in the other. For example, genes that make men more likely to be gay might make their female relatives more fertile or more attractive to mates. This “sexually antagonistic selection” could keep these genes circulating in the population because they provide a net benefit across both sexes.
What the Science Rules Out
Major medical organizations, including the American Medical Association, have taken clear positions on this topic. The AMA opposes the use of “conversion” or “reparative” therapy, reflecting the scientific consensus that sexual orientation is not something that can or should be changed through treatment. This position is grounded in decades of evidence showing that orientation is rooted in biology and developmental processes, not personal decisions.
Some older research suggested that childhood trauma or family dynamics could influence sexual orientation. While early traumatic experiences can affect many aspects of development and behavior, the scientific mainstream does not consider parenting styles, childhood experiences, or social environment to be primary causes of sexual orientation. The biological evidence, from genetics to prenatal hormones to immune responses, points consistently toward processes that begin before birth or in very early development, well before social influences could play a meaningful role.