Being transgender is not caused by any single factor. The best available evidence points to a combination of biological influences, including genetics, brain structure, and hormone exposure before birth, that together shape a person’s internal sense of gender. Major medical organizations including the Endocrine Society describe gender identity as reflecting “a complex interplay of biological, environmental, and cultural factors.” No one chooses to be transgender, and no parenting style or life event has been shown to cause it.
About 0.8% of U.S. adults and 3.3% of youth aged 13 to 17 identify as transgender, totaling over 2.8 million people. While researchers don’t have a complete picture of every mechanism involved, several biological pathways have strong supporting evidence.
Prenatal Hormones and Brain Development
The most well-supported explanation centers on hormone exposure during fetal development. In the womb, testosterone plays a key role in shaping the developing brain along sex-typical lines. In male fetuses, testosterone levels are elevated from roughly weeks 8 through 24 of pregnancy. This surge influences not just physical development but also how certain brain structures organize themselves. The prevailing theory is that, in some cases, the brain develops along a different path than the body, resulting in a gender identity that doesn’t match the sex assigned at birth.
Some of the strongest evidence for this comes from studying people with congenital adrenal hyperplasia (CAH), a genetic condition where the adrenal glands produce excess androgens. Girls with CAH who are exposed to unusually high levels of androgens before birth consistently show more male-typical play behavior, activity preferences, and, in a small percentage of cases, a male gender identity. This finding has been replicated in over a dozen studies across multiple countries. Importantly, boys with CAH who have similar medical treatment but normal prenatal androgen levels don’t show the same behavioral shifts, which suggests it’s the hormone exposure itself, not the medical condition or its treatment, driving the effect.
Children whose mothers were prescribed androgen-like hormones during pregnancy also show increased male-typical play, while those whose mothers took anti-androgenic hormones show the opposite. These findings reinforce that prenatal hormone levels influence gender-related development on a spectrum, with higher testosterone generally pushing toward more male-typical outcomes.
Brain Structure Differences
Several brain regions show measurable size and structural differences between men and women, and in transgender individuals, some of these regions more closely resemble those of their identified gender rather than their birth sex. One well-studied area is a small cluster of cells in the hypothalamus called the bed nucleus of the stria terminalis (BSTc). In the general population, this region is about 39% larger in men than in women and contains significantly more neurons. In transgender women (assigned male at birth), the BSTc has been found to be similar in size to that of non-transgender women. In a transgender man who was studied, the BSTc resembled that of non-transgender men.
Brain imaging studies using a technique called diffusion tensor imaging, which maps the wiring between brain regions, have found that transgender women who had not yet received any hormone treatment showed a pattern of brain connectivity that fell halfway between typical male and typical female patterns. Specific fiber bundles connecting different parts of the brain differed from both male and female controls. Researchers interpret this as evidence that certain brain pathways may not fully follow the masculinization process during development, even while the rest of the body does.
Genetic Influences
Genetics clearly play some role, though no single “transgender gene” exists. Twin studies have produced mixed but informative results. One widely cited study found that among identical twins (who share all their DNA), if one twin was transgender, the other was too about 39% of the time. Among fraternal twins, the concordance was 0%. That gap between identical and fraternal twins is a strong signal that genes contribute. However, a large register-based population study from Sweden found no concordant identical twin pairs, while 37% of different-sex fraternal twins both had gender dysphoria. The inconsistency across studies suggests that genetic influence is real but interacts heavily with other factors.
One specific genetic finding involves the androgen receptor gene, which determines how effectively the body responds to testosterone. This gene contains a repeating DNA sequence (called a CAG repeat), and longer repeats lead to weaker testosterone signaling. A study comparing transgender women to non-transgender male controls found that transgender women had significantly longer repeat lengths, meaning their cells may have responded less strongly to testosterone despite having typical male hormone levels. No significant associations were found for genes related to estrogen receptors or the enzyme that converts testosterone to estrogen.
Epigenetic Patterns
Beyond the DNA sequence itself, researchers have begun examining epigenetics: chemical modifications that sit on top of genes and influence whether they’re turned on or off. A study published in Frontiers in Neuroscience compared DNA methylation patterns (one type of epigenetic modification) between transgender and non-transgender individuals. The results were striking. In people assigned male at birth, 87 sites across the genome showed significantly different methylation levels between transgender women and non-transgender men. In people assigned female at birth, two sites showed significant differences. One of those sites, located on a gene called MPPED2, showed changes in both transgender men and transgender women, suggesting it may be relevant to gender identity regardless of birth sex.
These epigenetic differences were measured before any hormone treatment, ruling out the possibility that they were caused by medical transition. While this research is still in early stages with small sample sizes, it opens a pathway for understanding how gene activity, not just gene sequence, could shape gender identity during development.
What Doesn’t Cause It
Despite persistent misconceptions, no evidence supports the idea that parenting, social influence, or childhood experiences cause someone to be transgender. The biological evidence consistently points to factors established before or around birth. Studies of children with CAH are particularly instructive here: girls raised unambiguously as female, with surgical correction in infancy and strong social reinforcement of female identity, still sometimes develop a male gender identity during puberty if their androgen exposure was high and poorly controlled. Conversely, some children raised as one gender due to medical circumstances readily adopt the identity matching their hormonal and genetic profile once given the opportunity.
It’s also worth noting that being transgender is not itself a mental health condition. Gender incongruence, the experience of your gender not matching your birth sex, only becomes a clinical concern (called gender dysphoria) when it causes significant distress. The diagnosis is defined by the distress, not by the identity itself.
Why No Single Answer Exists
Gender identity emerges from the interaction of multiple biological systems: genes that shape hormone receptors, prenatal hormone levels that organize the brain, epigenetic switches that fine-tune gene activity, and brain structures that develop along sex-differentiated lines. None of these factors alone is sufficient to determine someone’s gender identity, and the specific combination likely varies from person to person. This is consistent with what researchers see across many complex human traits, where dozens or hundreds of small biological influences combine with environmental factors to produce individual variation. The Endocrine Society’s summary remains the most accurate framing: biologic factors, in addition to environmental factors, contribute to this fundamental aspect of human development.