Sexual orientation is partly genetic, but there is no single “gay gene.” The largest genetic study on the topic, published in Science in 2019 and involving nearly half a million people, found that all common genetic variants combined account for 8 to 25% of the variation in same-sex sexual behavior. The rest comes from a mix of other biological factors, prenatal environment, and influences researchers still don’t fully understand.
The short answer: genes play a real role, but they’re one piece of a more complex picture. Here’s what the science actually shows.
What the Largest Genetic Study Found
The 2019 study, led by Andrea Ganna and an international team of researchers, analyzed genetic data from 477,522 people in the UK Biobank and 23andMe. It was the first study with enough statistical power to identify specific genetic variants linked to same-sex behavior. Five variants passed the strictest thresholds for significance, but each one had a tiny individual effect. The researchers concluded that same-sex sexual behavior “is influenced by not one or a few genes but many.”
Perhaps the most striking finding was what the study ruled out. There is no single genetic switch that determines sexual orientation. The genetic architecture looks more like height or personality, where thousands of small genetic contributions add up. Even taken together, all measurable genetic variants explained only a modest share of the variation, somewhere between 8 and 25% depending on the statistical method used.
The study also found that the genetic influences on same-sex behavior in men and women only partially overlap. The correlation between the sexes was about 0.63, meaning some genetic factors are shared but others differ. And the genetic effects distinguishing people who have ever had a same-sex partner from those who haven’t were not the same as those influencing the proportion of same-sex partners among nonheterosexual people. In other words, sexual orientation doesn’t map onto a single genetic continuum.
Twin Studies and Heritability
Before large-scale genetic studies were possible, researchers relied on twins to estimate how heritable sexual orientation might be. Identical twins share virtually all their DNA, while fraternal twins share about half. If identical twins are more likely to share the same sexual orientation than fraternal twins, that points to a genetic contribution.
That’s exactly what decades of twin research have found. Identical twins are significantly more concordant for sexual orientation than fraternal twins. But the concordance is far from 100%. If one identical twin is gay, the other is more likely to be gay than a non-twin sibling would be, but it’s not a certainty. This pattern tells us genetics matters, but it also tells us that something beyond DNA sequence is involved.
Prenatal Hormones and Brain Development
One of the strongest lines of biological evidence points to the hormone environment in the womb. In virtually all mammals, exposure to testosterone and related hormones during critical windows of fetal development shapes brain structures tied to sexual behavior. The evidence supports a similar role in humans: prenatal testosterone exposure influences sex-typed interests in childhood and, for at least some individuals, sexual orientation later in life.
This doesn’t mean testosterone levels alone determine orientation. Other factors, including direct genetic effects and maternal conditions during pregnancy, also appear to play a role. But the prenatal hormone environment helps explain why sexual orientation feels innate to most people. It’s shaped during brain development, long before conscious experience.
Neuroscience research adds a structural dimension. In a landmark 1991 study, neuroscientist Simon LeVay examined a small cluster of cells in the hypothalamus called INAH-3, a region involved in reproductive behavior. He found that this cell group was more than twice as large in heterosexual men as in homosexual men, and roughly similar in size between homosexual men and women. While the study had limitations (small sample, postmortem tissue), it was early evidence that sexual orientation has a measurable biological substrate in the brain.
The Older Brother Effect
One of the most consistently 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 roughly 33%. This effect has been confirmed across cultures, including non-Western populations, and it only applies to biological brothers, not stepbrothers or adoptive brothers raised in the same household.
The leading explanation is immunological. During pregnancy with a male fetus, the mother’s immune system may develop antibodies against proteins produced by genes on the Y chromosome, particularly a molecule called H-Y antigen that plays a role in male fetal development. With each successive male pregnancy, the maternal immune response grows stronger. These antibodies may cross the placental barrier and affect brain development in regions tied to sexual orientation, including the anterior hypothalamus. Research has confirmed that mothers of multiple boys do develop increasingly strong immune responses to Y-linked proteins, which supports the biological plausibility of this mechanism.
Epigenetics: Beyond the DNA Sequence
Epigenetics offers a way to bridge the gap between the modest genetic contribution and the clearly biological nature of sexual orientation. Epigenetic marks are chemical tags, like methyl groups attached to DNA, that regulate how genes are expressed without changing the underlying genetic code. These marks can be influenced by the hormonal environment during fetal development, and they can differ dramatically even between identical twins.
One theoretical model proposes that epigenetic marks normally help calibrate a fetus’s sensitivity to sex hormones. In most cases, female fetuses develop marks that make their cells less responsive to circulating androgens, while male fetuses develop marks that enhance androgen sensitivity. Occasionally, these marks may carry over across generations or form atypically during development, leading to a mismatch between genetic sex and the brain’s response to hormones. Researchers have observed DNA methylation differences of up to 54% at certain gene sites between individuals at birth, showing just how much variation epigenetic programming can create. This area of research is still developing, but it may help explain how identical twins with the same DNA can differ in sexual orientation.
Why These Genes Persist in the Population
If genes contributing to same-sex attraction reduce the likelihood of reproduction, natural selection should have weeded them out over time. The fact that they persist has puzzled evolutionary biologists, and two leading hypotheses offer explanations.
The first is the kin selection hypothesis, sometimes called the “gay uncle” theory. It proposes that individuals who don’t reproduce themselves can still promote their genes by investing in the survival of nieces and nephews, who share a portion of their DNA. Research on Samoan fa’afafine, biological males who take on a feminine social role, found that they are indeed more altruistically inclined toward their kin than heterosexual men are, lending some support to this idea.
The second is the sexual antagonism hypothesis, which may have stronger empirical backing. It suggests that genetic variants associated with male homosexuality provide a reproductive advantage to women who carry them. Studies have found that maternal relatives of gay men tend to have more children than average. If these shared genetic variants boost female fertility, they would be maintained in the population even if they sometimes reduce male reproduction. The net effect on gene transmission stays positive.
What the Scientific Community Says
The American Psychological Association states that there is no consensus on the exact reasons a person develops a particular sexual orientation, and no findings show that orientation is determined by any single factor. Most scientists think nature and nurture both play complex roles. Critically, most people experience little or no sense of choice about their sexual orientation.
What the evidence does clearly establish is that sexual orientation has deep biological roots. Genes contribute meaningfully but modestly. Prenatal hormones shape brain development during pregnancy. Immune responses, epigenetic variation, and brain structure all add layers. The picture that emerges is not one of genetic determinism or personal choice, but of a trait shaped by multiple biological processes, most of them occurring before birth.