Sexual orientation is influenced by genetics, but no single gene determines whether someone is gay, straight, or bisexual. The largest genetic study on the topic, involving nearly half a million people, found that all measured genetic variants together explained only about 8 to 25 percent of the variation in same-sex sexual behavior. The rest comes from a tangle of prenatal hormones, immune responses, epigenetic changes, and environmental factors that researchers are still working to untangle. The honest picture is not “nature versus nurture” but a complex developmental process in which biology plays a substantial role without being the whole story.
What Twin Studies Reveal
The strongest early evidence that sexual orientation has a biological basis came from studying twins. Identical twins share virtually all their DNA, while fraternal twins share roughly half, making twin pairs a natural experiment for separating genetic from environmental influences. In one study of 61 twin pairs, identical twins showed a concordance rate of about 66 percent for homosexual orientation, meaning that if one twin was gay, the other was too about two-thirds of the time. Fraternal twins had a concordance rate of roughly 30 percent.1PubMed. Homosexual orientation in twins: a report on 61 pairs and three triplet sets Decades of similar research have consistently shown that same-sex sexual behavior has a substantial heritable component.2PubMed. Emerging insights into the genetics and evolution of human same-sex sexual behavior
But that 66 percent concordance rate, while striking, also contains a revealing gap. If sexual orientation were entirely genetic, identical twins would match 100 percent of the time. The fact that roughly a third of identical twin pairs have different sexual orientations tells us that genes are not the full explanation.3PubMed Central. Sexual Orientation in Twins: Evidence That Human Sexual Identity May Be Determined Five Days Following Fertilization Something else, whether it happens in the womb, during early development, or through some other pathway, must also contribute. This gap is what drives most of the ongoing research in the field.
What the Genome Actually Shows
For years, media headlines promised the discovery of a “gay gene.” That never happened, and the science now makes clear why it never will. In 2019, researchers published the largest genome-wide association study on same-sex sexual behavior, analyzing genetic data from 477,522 individuals. They identified five specific locations in the genome that were significantly associated with same-sex sexual behavior. But individually, each of these had a tiny effect. When the researchers added up the influence of every genetic variant they could measure, the total explained only about 8 to 25 percent of the variation in same-sex behavior.4PubMed Central. Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior
Two findings from that study are worth sitting with. First, the genetic variants associated with same-sex behavior only partially overlapped between men and women, suggesting that the biology underlying sexual orientation differs somewhat by sex. Second, even with all the genetic data in hand, it was not possible to meaningfully predict any individual’s sexual behavior from their DNA. This is not a case where a genetic test could ever tell you who is or will be gay. Sexual orientation is what geneticists call a complex trait, shaped by many genes with small effects, much like height or personality, but even harder to pin down because non-genetic factors play a bigger role.
Prenatal Hormones and Development in the Womb
One of the most well-supported biological threads in sexual orientation research involves hormones that a fetus is exposed to before birth. The basic idea is that sex hormones, particularly androgens like testosterone, shape the developing brain in ways that influence who a person will later be attracted to. This is not about hormone levels in adulthood. It is about the hormonal environment during a critical window of brain development.
Researchers cannot ethically measure fetal hormone levels directly in humans, so they rely on indirect markers. One widely studied marker is the ratio between the length of the second and fourth fingers, known as the digit ratio. This ratio is influenced by prenatal androgen exposure: more androgen exposure tends to produce a lower ratio. A 2025 meta-analysis found that gay women tend to have lower, more male-typical digit ratios on both hands compared to straight women, while gay men tend to have slightly higher, more female-typical ratios compared to straight men.5PubMed Central. Sexual orientation is associated with 2D:4D finger length ratios in both sexes: an updated and expanded meta-analysis The effects are small but consistent across many studies, and they point in the direction you would expect if prenatal hormone exposure partly shapes sexual orientation.
Earlier research found similar patterns, with both gay men and gay women showing lower digit ratios than their heterosexual counterparts, though the direction and magnitude of the effect have varied across individual studies.6PubMed. Sexual orientation and the 2nd to 4th finger length ratio: evidence for organising effects of sex hormones or developmental instability? The inconsistency across studies is itself informative: it tells us that prenatal hormones are one factor among many, not a switch that flips orientation in a predictable direction.
A Surprising Clue From the Inner Ear
One of the more unexpected pieces of evidence for prenatal hormone influence comes not from the brain but from the ear. The inner ear produces faint sounds called otoacoustic emissions, and these emissions are sexually dimorphic from birth: newborn girls produce stronger emissions than newborn boys, a difference that persists throughout life. This sex difference is thought to reflect prenatal androgen exposure’s effect on the developing auditory system.
When researchers compared these emissions across women of different sexual orientations, they found that gay and bisexual women produced weaker emissions than straight women, falling in between heterosexual females and heterosexual males.7PubMed. Comparison of the auditory systems of heterosexuals and homosexuals: click-evoked otoacoustic emissions The same pattern held for spontaneous otoacoustic emissions: gay and bisexual women had fewer and weaker emissions than straight women.8PubMed. Spontaneous otoacoustic emissions in heterosexuals, homosexuals, and bisexuals Researchers interpret these findings as evidence that the auditory systems, and likely the brains, of some nonheterosexual women were partially masculinized by prenatal androgen exposure.9PubMed Central. Sexual orientation and the auditory system
What makes the ear data compelling is that it connects sexual orientation to a measurable physiological difference that is present from birth and has nothing to do with social learning or personal choice. The ear does not “decide” to develop differently based on upbringing. It develops the way it does because of hormonal conditions in the womb.
The Older Brother Effect
One of the most reliably replicated findings in sexual orientation research is that men with more older brothers are somewhat more likely to be gay. Each additional older brother increases the odds by a measurable amount. This is called the fraternal birth order effect, and it applies specifically to biological older brothers, not stepbrothers or adopted brothers, and only to men.
The leading explanation is immunological. The hypothesis proposes that when a woman is pregnant with a male fetus, her immune system may develop antibodies against certain proteins produced by genes on the Y chromosome. These proteins, called H-Y antigens, play roles in male brain development. With each subsequent male pregnancy, the mother’s immune response to these proteins could grow stronger, and the antibodies may cross the placental barrier and affect how the next male fetus’s brain develops.10PubMed. Fraternal birth order and the maternal immune hypothesis of male homosexuality
In 2017, researchers found direct biological support for this idea. Mothers of gay sons, particularly those with older brothers, showed higher levels of antibodies against a specific Y-linked protein called NLGN4Y, which is involved in the development of connections between brain cells.11PubMed Central. Male homosexuality and maternal immune responsivity to the Y-linked protein NLGN4Y This was the first time researchers had identified a plausible molecular mechanism behind the birth order effect. A comprehensive review of the evidence concluded that while no study had yet demonstrated the exact prenatal mechanism step by step, the immunological explanation remains the most plausible one available.12PubMed. Sexual orientation, fraternal birth order, and the maternal immune hypothesis: a review
The older brother effect cannot explain most cases of male homosexuality, since plenty of gay men are firstborns or have no brothers at all. But it demonstrates how a non-genetic biological mechanism can influence sexual orientation in a measurable, replicable way.
Brain Differences Between Gay and Straight Men
In 1991, neuroscientist Simon LeVay reported that a small cluster of cells in the hypothalamus, a brain region involved in regulating basic drives, differed in size between heterosexual and homosexual men. The cell group known as INAH 3 was more than twice as large in heterosexual men as in homosexual men, and the size in gay men was comparable to that seen in women.13PubMed. A difference in hypothalamic structure between heterosexual and homosexual men This was a landmark finding because it suggested a structural basis in the brain for sexual orientation.
A later study partially replicated the finding, observing a trend for INAH 3 to be smaller in gay men, but also found no difference in the actual number of neurons within that region based on sexual orientation.14PubMed. The interstitial nuclei of the human anterior hypothalamus: an investigation of variation with sex, sexual orientation, and HIV status So the picture is more nuanced than “gay men have female-sized brain structures.” The difference may be more about how cells are arranged or connected than about raw cell count.
Functional brain imaging adds another dimension. When researchers exposed heterosexual men, heterosexual women, and homosexual men to a compound derived from male sweat thought to function as a pheromone, gay men responded like heterosexual women: the compound activated areas in the hypothalamus linked to sexual behavior rather than being processed as an ordinary smell. Heterosexual men did not show this hypothalamic activation to the male-derived compound.15PubMed Central. Brain response to putative pheromones in homosexual men These results do not prove that brain differences cause sexual orientation rather than reflecting it, but they do show that the neurological underpinnings of attraction differ measurably between gay and straight men.
Epigenetics and the Missing Heritability
The gap between the strong heritability seen in twin studies and the relatively modest genetic signal found in genome-wide studies has a name: missing heritability. One promising candidate for filling that gap is epigenetics, the system of chemical modifications to DNA that can change how genes are expressed without altering the genetic code itself.
A theoretical model published in 2012 proposed that sex-specific epigenetic marks, which normally help a female fetus resist the masculinizing effects of androgens or help a male fetus respond to them, might occasionally carry over from one generation to the next. If a father’s epi-marks that dampened androgen sensitivity were transmitted to a daughter, no unusual effect would occur. But if those same marks were transmitted to a son, they could feminize aspects of brain development, including sexual orientation. The reverse process could masculinize a daughter’s development.16PubMed. Homosexuality as a consequence of epigenetically canalized sexual development
This model is elegant and biologically plausible, but it remains largely theoretical. Direct evidence that specific epigenetic marks are transmitted across generations and influence sexual orientation in humans has not yet been demonstrated. Still, the epigenetic framework helps explain something that pure genetic models struggle with: how sexual orientation can run in families, show up more often in identical twins, and yet not map cleanly onto any particular stretch of DNA.
Why Hasn’t Evolution Eliminated Homosexuality?
If same-sex attraction reduces the likelihood of reproduction, and traits that reduce reproduction tend to be weeded out over generations, why do genes influencing homosexuality persist in the human population? This question has generated several competing evolutionary hypotheses, and the evidence so far does not clearly favor one.
The kin selection hypothesis proposed that gay individuals might compensate for reduced direct reproduction by investing extra resources in their nieces and nephews, thereby helping shared genes survive. Testing this idea in Western populations has not gone well. Studies found that gay men were not more generous toward relatives than straight men, and on some measures were slightly more emotionally distant from family members.17Evolution and Human Behavior. Is male homosexuality maintained via kin selection? A separate community-based study also found little support for the kin selection theory.18PubMed. An empirical test of the kin selection hypothesis for male homosexuality This does not mean the idea is impossible in every cultural context, but it has not held up under direct testing in the settings where it has been examined.
A more promising explanation is sexually antagonistic selection. The idea here is that genetic variants that contribute to male homosexuality might increase reproductive fitness when they appear in women. Italian family studies provided early support for this: female relatives of gay men tended to have more children than female relatives of straight men. Modeling work has shown that a genetic architecture involving at least one gene on the X chromosome, where expression benefits females while disadvantaging males reproductively, can account for the known data.19PubMed Central. Sexually Antagonistic Selection in Human Male Homosexuality Under this model, homosexuality in men persists because the same genetic variants boost fertility in women enough to compensate. The math works, though more research is needed to identify the specific genes involved.
Same-sex sexual behavior also appears widely across the animal kingdom, documented in hundreds of species from insects to mammals.20PubMed. Same-sex sexual behavior and evolution The sheer prevalence across species suggests that the evolutionary calculus is more complicated than a simple “reproduce or die out” model. Same-sex behavior may serve social bonding functions, may be a byproduct of genes selected for other reasons, or may simply not carry enough of a reproductive penalty to be strongly selected against.
How Sex and Gender Nonconformity Interact With Genetic Influence
The genetic and environmental contributions to sexual orientation do not appear to be fixed quantities; they may vary depending on other traits. A study examining the relationship between childhood gender nonconformity and sexual orientation found that as gender nonconformity increased, the overall variation in sexual orientation decreased. This narrowing of variation was driven primarily by a decrease in the influence of individual-specific environmental factors rather than changes in genetic influence.21PubMed Central. Genetic and Environmental Influences on Sexual Orientation: Moderation by Childhood Gender Nonconformity and Early-Life Adversity
Put simply, among people who were very gender nonconforming as children, sexual orientation seems to be more tightly channeled, with less room for environmental variation to push in different directions. Among people who were more gender-typical as children, environmental factors appear to play a relatively larger role in the variation. This does not mean that gender nonconformity causes homosexuality or vice versa. It means the two traits share some developmental roots, likely including the prenatal hormone environment discussed earlier, and that the balance of genetic and environmental influences on orientation is not identical for everyone.
What “Choice” Gets Wrong
The research reviewed here converges on a point that matters beyond the laboratory: sexual orientation is not a conscious decision. The biological influences begin before birth, in the hormonal environment of the womb, in the genetic variants a person inherits, in the immune responses of a pregnant woman’s body. Brain structures associated with attraction show measurable differences that track with orientation. Physiological markers in the inner ear, which no amount of willpower can alter, differ between heterosexual and nonheterosexual women. None of this is accessible to deliberate choice.
This does not mean science has identified a single biological cause. It has not, and it almost certainly will not, because complex traits do not work that way. Sexual orientation emerges from a web of factors that differ from person to person. For one man, the fraternal birth order effect may have played a role. For another, the genetic variants identified in the 2019 genome-wide study may have contributed. For a third, epigenetic factors or prenatal hormone patterns may have been more influential. The mixture is different in each case, and researchers are still far from being able to assign relative weights to each factor for any individual.
Differences Between Men and Women
Much of the research on sexual orientation has focused on men, and the findings do not always transfer cleanly to women. The fraternal birth order effect, for instance, has been documented only in men. The brain imaging studies on hypothalamic responses to putative pheromones tested heterosexual and homosexual men specifically. The 2019 genome-wide study explicitly noted that the genetic variants associated with same-sex behavior only partially overlapped between men and women.4PubMed Central. Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior
Where data on women does exist, it sometimes points in a different direction. The digit ratio and otoacoustic emission findings are strongest and most consistent for women, suggesting that prenatal androgen exposure plays a particularly clear role in female sexual orientation. Some researchers have argued that female sexuality may be more fluid and more responsive to social and experiential factors than male sexuality, but this is a contested claim and the evidence is mixed. What is clear is that the biology of sexual orientation is not identical across sexes, and findings from studies of men should not be assumed to apply to women without separate evidence.
The Inner Ear as a Window Into Prenatal Development
The otoacoustic emission findings deserve a closer look because they illustrate a broader principle. The inner ear and the brain develop under the influence of the same prenatal hormone environment, but the ear is far easier to measure than the brain. You cannot ethically biopsy a living person’s hypothalamus, but you can place a tiny microphone in the ear canal and measure the sounds that come back. The fact that these emissions differ by sexual orientation in women provides a kind of indirect window into what was happening hormonally during fetal development.
Importantly, the ear data comes from adult participants whose ear development was fixed long before any social or cultural influences on sexuality could have played a role. The cochlea finishes developing prenatally. This makes otoacoustic emissions one of the clearest pieces of evidence that at least some aspect of sexual orientation, particularly in women, is shaped before birth by hormonal conditions that also affect non-sexual body structures.