Being transgender appears to result from a combination of biological factors, primarily the way the brain develops before birth. About 1% of people aged 13 and older in the United States identify as transgender, and the best available evidence points to prenatal hormone exposure, genetics, and brain development as the core influences. There is no single “cause,” but the science increasingly shows that gender identity has deep biological roots.
How Gender Identity Develops in the Brain
During fetal development, the brain and the body develop along sexual lines at different times and through partially independent processes. The genitals typically differentiate during the first trimester, while the brain regions involved in gender identity are shaped later in pregnancy. This means hormonal shifts or variations during a critical window can lead the brain to develop along a different path than the rest of the body.
Brain imaging studies have found that several specific structures in transgender individuals more closely resemble those of their identified gender than their sex assigned at birth. These include a region deep in the brain called the bed nucleus of the stria terminalis, which plays a role in sexual behavior and stress responses, as well as parts of the hypothalamus, the insula, and areas around the central sulcus. A 2022 review in the Journal of Clinical Medicine described this pattern as the brain being “shifted towards gender identity” in transgender women. The differences are subtle and statistical rather than absolute, but they consistently appear across multiple research groups using different methods.
The Role of Prenatal Hormones
The strongest line of evidence comes from studying people born with conditions that alter their hormone exposure before birth. Testosterone and related hormones during fetal development don’t just shape the body; they organize brain circuits in ways that influence behavior, preferences, and identity later in life. These effects are often called “organizational” because they create lasting structural changes rather than temporary ones.
Women born with congenital adrenal hyperplasia (CAH), a condition that exposes a female fetus to unusually high levels of androgens, provide a natural test case. About 1% to 3% of women with CAH eventually transition to living as men. That’s a small percentage in absolute terms, but it’s dramatically higher than the rate in the general population, strongly suggesting that prenatal androgen exposure pushes gender identity in a male-typical direction.
The reverse pattern is equally telling. People born with complete androgen insensitivity syndrome (CAIS) are genetically male (XY chromosomes) but their cells cannot respond to testosterone at all. Despite having male chromosomes, they almost always develop a female gender identity. Their brains, like their bodies, developed without effective androgen signaling. Taken together, these natural experiments show that the hormonal environment during brain development is a powerful influence on the gender identity that eventually emerges.
Genetics and Heritability
Twin studies offer the clearest window into whether genes play a role, and the answer is yes, though genes alone don’t determine the outcome. One widely cited study found that when one identical twin was transgender, the other twin was also transgender 39.1% of the time. For non-identical twins, that rate dropped to 0%. Since identical twins share all their DNA and non-identical twins share roughly half, a gap that large points to a significant genetic component.
Estimates of heritability vary depending on the study and the population. One analysis placed it at 62%, while another found heritability of 41% for individuals assigned female at birth and much lower for those assigned male. These inconsistencies likely reflect different sample sizes and methods, but the overall pattern is clear: genes contribute meaningfully without being the whole story.
Researchers have also looked at specific genes that might be involved. A study of 112 transgender women and 258 cisgender men found that the transgender women had longer repeat sequences in the androgen receptor gene. Longer repeats in this gene are associated with reduced androgen signaling, which could result in a brain that’s less masculinized during development, even when testosterone levels are normal. This finding fits neatly with the prenatal hormone theory: it’s not just how much testosterone is present, but how sensitive the brain is to it.
When Gender Identity Becomes Apparent
Children typically begin expressing a sense of gender between ages 2 and 4, and for many transgender people, the feeling of incongruence starts early. A large survey of over 27,000 transgender adults found that about 59% realized their gender identity differed from societal expectations based on their assigned sex by age 10 or younger. The remaining 41% came to that realization during adolescence or adulthood.
That nearly equal split is important. It challenges the idea that being transgender always looks the same or follows a single timeline. Some people have vivid memories of gender incongruence from their earliest years. Others may not have the language, context, or safety to recognize or articulate what they’re experiencing until much later. The underlying identity appears to be stable, but the awareness of it varies widely.
This Is Not a Modern Phenomenon
Gender-diverse people have existed across cultures and throughout recorded history, long before modern medicine or social media. More than 150 pre-colonial Native American tribes recognized third-gender roles in their communities. The Zuni people had the concept of Lhamana, individuals assigned male at birth who lived as women and held respected social positions. The Crow nation honored people they called baté, who moved between traditionally male and female roles and were often regarded as bridges between genders.
Similar roles appear across the globe. The Hijra of South Asia have been documented for centuries. Muxes in Zapotec culture in southern Mexico, Bakla in pre-colonial Philippines, and feminine spiritual leaders in pre-colonial West Africa all represent culturally recognized gender identities that don’t map onto a strict male-female binary. The language and social structures differ, but the underlying reality is the same: in every era and region where historians have looked, some people’s gender identity has differed from their sex assigned at birth.
Putting the Pieces Together
No single factor makes someone transgender. The most supported model is that gender identity is shaped primarily during fetal brain development, through a combination of genetic predisposition, hormone levels, and how sensitive the developing brain is to those hormones. Epigenetic factors, which are chemical modifications to DNA that affect how genes are expressed without changing the genetic code itself, may also play a role, though research in this area is still in early stages.
What the evidence does not support is the idea that being transgender is a choice, a phase, or the result of upbringing. The biological evidence, from brain structure to prenatal hormones to twin concordance, consistently points to factors that are set before birth or very early in development. The cross-cultural and historical record reinforces this: transgender people have always existed, in societies with vastly different attitudes toward gender, long before any of the social or political dynamics that sometimes frame the modern conversation.