There is no single thing that “makes” a person gay. Sexual orientation emerges from a combination of genetic, hormonal, and developmental factors that interact in ways science is still working to fully map. What is clear is that it is not a choice, not the result of parenting, and not caused by any one gene. Most people experience little or no sense of choice about who they are attracted to, and the biological roots of sexual orientation begin long before birth.
Genetics Play a Role, but No “Gay Gene” Exists
One of the largest genetic studies ever conducted on sexual orientation, published in the journal Science in 2019, analyzed the DNA of nearly half a million people. The researchers found that common genetic variants account for roughly 8 to 25% of the variation in same-sex sexual behavior. Broad-sense heritability, which captures additional genetic effects, was estimated at about 32%. That means genetics matter, but they are far from the whole story.
The study identified five specific locations in the genome linked to same-sex behavior, spread across chromosomes 4, 7, 11, 12, and 15. Some of these signals differed between men and women. But each individual variant had an extremely small effect. As one of the lead researchers put it: “There is no ‘gay gene.’ Rather, nonheterosexuality is in part influenced by many tiny genetic effects.” This is similar to how height or personality traits work. Hundreds or thousands of genetic variants each nudge the outcome slightly, and none of them is decisive on its own.
Twin studies reinforce the genetic picture. In one well-known study, when one identical twin was gay, the other was also gay about 52% of the time. For fraternal twins, that rate dropped to 22%, and for adoptive (non-biological) brothers raised in the same home, it was just 11%. If sexual orientation were purely genetic, identical twins would match 100% of the time. If it were purely environmental, adoptive brothers would match at a much higher rate. The gradient from identical to fraternal to adoptive siblings points to a significant genetic contribution layered on top of other biological factors.
Hormones in the Womb Shape Development
Before birth, the hormonal environment in the uterus appears to influence sexual orientation. The key players are androgens (a group of hormones that includes testosterone) and estrogens. During critical windows of fetal development, these hormones help shape the brain’s structure and wiring, including the circuits involved in sexual attraction.
One line of evidence comes from finger length ratios. The relative length of your index finger compared to your ring finger is partly set by prenatal hormone exposure: more androgen exposure in the womb produces a relatively longer ring finger. A large meta-analysis found that gay women tend to have slightly more male-typical finger ratios (suggesting higher prenatal androgen exposure), while gay men tend to have slightly more female-typical ratios (suggesting lower androgen or higher estrogen exposure). The differences are small and far from diagnostic, but the pattern is consistent across dozens of studies and aligns with the idea that the prenatal hormonal environment nudges sexual orientation.
Additional evidence comes from medical conditions that alter prenatal hormone levels. Women with congenital adrenal hyperplasia, a condition that exposes a female fetus to elevated androgens, are more likely to report same-sex attraction as adults. Conversely, genetic males with androgen insensitivity syndrome, where the body cannot respond to testosterone, typically develop a female gender identity and are attracted to men. These cases are not the norm for most gay or lesbian people, but they demonstrate that prenatal hormones can directly influence the direction of sexual attraction.
The Birth Order Effect in Males
One of the most consistently replicated findings in this area is the fraternal birth order effect: each older biological brother a man has increases his likelihood of being gay by roughly 33%. This effect applies only to biological brothers, not stepbrothers, and only in males.
The leading explanation is immunological. When a woman carries a male fetus, her immune system is exposed to proteins produced by the Y chromosome (called H-Y antigens). With each successive male pregnancy, her immune response to these proteins may grow stronger. The resulting antibodies could cross the placenta and subtly alter brain development in later-born sons, particularly the areas involved in sexual attraction. This is still a hypothesis, but the birth order pattern itself is one of the most robust findings in sexual orientation research.
Epigenetics: Same DNA, Different Expression
Identical twins share the same DNA sequence, yet one can be gay while the other is straight. This points to epigenetics, the system of chemical tags on DNA that control which genes are active and which are silent without changing the genetic code itself. Two of the most important epigenetic mechanisms are DNA methylation (adding chemical groups that typically silence a gene) and histone modification (altering the proteins that DNA wraps around, making genes more or less accessible).
Research in animals has shown that manipulating these epigenetic marks during critical developmental periods can disrupt the typical sexual differentiation of the brain. In rodents, interfering with either DNA methylation or histone modification changes the development of brain structures that normally differ between males and females. In humans, studies have found that males and females show different patterns of epigenetic marks across hundreds of genes, and intriguingly, many of these differences don’t correspond to differences in gene expression. This suggests that male and female brains may use different epigenetic strategies to regulate the same genes, a finding that opens the door to understanding how subtle epigenetic shifts could influence traits like sexual orientation.
Epigenetics may also explain how environmental factors in the womb, such as hormone levels, stress, or immune activity, get translated into lasting changes in brain development. Rather than altering DNA itself, these influences could leave epigenetic marks that shape how the brain responds to sexual cues later in life.
Brain Structure Differences
Post-mortem studies have found structural differences in a tiny region of the hypothalamus called INAH3, which is involved in reproductive behavior. This cluster of neurons is typically larger in heterosexual men than in women. Some studies have found that it tends to be smaller in gay men, closer to the size observed in women, though results have been mixed. One study confirmed a trend toward smaller volume in gay men but found no actual difference in the number of neurons, suggesting the distinction may be about how densely the neurons are packed rather than how many exist.
These brain differences do not tell us whether they are a cause or a consequence of sexual orientation, and the sample sizes in post-mortem research are necessarily small. But they add to the broader picture that sexual orientation has a biological basis in the brain’s physical structure.
Parenting and Social Environment Do Not Determine Orientation
Despite decades of searching, no credible evidence has emerged that parenting style, childhood experiences, or social environment determine whether someone is gay or straight. Children raised by same-sex parents are no more likely to be gay than children raised by opposite-sex parents. The American Psychological Association’s official position states that no findings permit scientists to conclude sexual orientation is determined by any particular factor, but emphasizes that most people experience no sense of choice about it. The scientific consensus is that nature and nurture both play complex roles, with biological factors carrying the most explanatory weight.
Why These Traits Persist in Evolution
If sexual orientation has a genetic component, a natural question is why genes associated with same-sex attraction haven’t disappeared over evolutionary time. Several hypotheses address this. The kin selection hypothesis proposes that gay individuals historically boosted the survival of nieces, nephews, and other close relatives by providing additional childcare and resources, indirectly passing on shared genes. The sexual antagonism hypothesis offers a different angle: genes linked to male homosexuality may increase fertility in female relatives. Studies have found that mothers and maternal aunts of gay men tend to have more children than average, suggesting these genetic variants give women a reproductive advantage even as they influence orientation in men.
A broader view holds that the genetic diversity underlying sexual orientation may itself be adaptive for human populations, contributing to complex social structures and cooperative behavior. None of these hypotheses is fully proven, but together they explain why the genetics of sexual orientation are maintained rather than selected out.
The Bottom Line on Causation
Sexual orientation is shaped by many small genetic influences, prenatal hormone exposure, epigenetic regulation, and possibly immune factors during pregnancy. These biological threads weave together differently in every person, which is why there is no single cause and no reliable way to predict orientation before it naturally emerges. What the evidence consistently rules out is the idea that being gay is a lifestyle decision or the product of upbringing. It is a natural variation in human biology, rooted in processes that begin before birth.