Transgender people exist because gender identity, like many human traits, is shaped by a combination of biological factors that begin before birth. The best available science points to genetics, prenatal hormone exposure, and brain development as the primary drivers. About 1% of people aged 13 and older in the United States identify as transgender, roughly 2.8 million people, and gender-diverse individuals have been documented across cultures and throughout recorded history.
Gender Identity Develops Before Birth
The key to understanding why trans people exist lies in how the brain and body develop on separate timelines during pregnancy. Sexual differentiation of the genitals happens during the first trimester, but the brain’s sexual differentiation occurs later, during the second half of pregnancy and into early infancy. These two processes are both influenced by hormones, particularly testosterone, but because they happen at different times, they can be influenced by different hormonal conditions. The result is that a person’s brain can develop along one path while their body develops along another.
This isn’t just theoretical. In mammals, testosterone and the hormones produced from it influence cell survival, neurite outgrowth, and neurochemical development in brain regions that differ between males and females. When researchers study children with congenital adrenal hyperplasia (CAH), a condition that exposes a fetus to unusually high levels of androgens, they find that girls with CAH consistently show more male-typical behavior. Researchers have ruled out other aspects of the condition as the cause: boys with CAH have the same corticosteroid abnormalities and treatments but don’t show increased masculinization, because their prenatal androgen levels are already typical. Additional evidence comes from studies of children whose mothers took androgenic or anti-androgenic hormones during pregnancy, which shifted behavior in the expected directions.
Brain Structure Differences
Neuroimaging research has found that transgender people’s brains show structural patterns shifted toward their gender identity rather than matching their sex assigned at birth. Studies have identified these shifts in multiple brain regions, including areas involved in body perception, self-awareness, and emotional processing: the bed nucleus of the stria terminalis, the insula, the posterior cingulate, the area around the central sulcus, and regions of the occipital cortex, among others.
One particularly telling line of research involves white matter microstructure. A diffusion tensor imaging study of transgender men before any hormone treatment found that their white matter patterns already differed from those of cisgender women and shifted toward a male-typical pattern. This is significant because it shows these brain differences aren’t a result of taking hormones. They exist before any medical intervention, suggesting they reflect the way the brain originally developed.
Genetics Play a Role
Twin studies offer one of the clearest windows into genetic influence. Researchers compare identical twins (who share 100% of their DNA) with fraternal twins (who share about 50%) to estimate how much genetics contributes to a trait. Systematic reviews of this research have found higher concordance rates for transgender identity in identical twins than in fraternal twins, indicating a meaningful genetic component. Molecular genetic studies have also identified variants in genes involved in sex hormone signaling that may predispose individuals to a transgender identity.
That said, genetics alone don’t determine gender identity. If they did, identical twins would always match. The current scientific understanding is that multiple genes each contribute a small amount of influence, interacting with the prenatal hormonal environment to shape how the brain develops. This is similar to how many complex human traits work: no single gene is responsible, but genetic variation creates a range of possible outcomes.
Gender Diversity Across Cultures and History
If transgender identity were simply a modern Western phenomenon, it would be difficult to argue for a biological basis. But gender-diverse people appear in cultures around the world, independently of one another, across thousands of years.
- Hijra in South Asia appear in Hindu religious texts and have held recognized spiritual roles, performing rituals at weddings and births, for centuries.
- Two-spirit people have existed across many Indigenous North American cultures long before European contact. Though the umbrella term was coined in 1990, the traditions it describes are ancient, and two-spirit individuals have held respected roles as healers, ceremonial leaders, and artisans.
- Muxes among the Zapotec people of Oaxaca, Mexico, are people assigned male at birth who embrace a feminine identity, embedded within Indigenous cultural tradition.
- Bakla in the Philippines historically encompassed aspects of both masculinity and femininity, and bakla individuals often served as community leaders.
- Bissu among the Bugis of South Sulawesi, Indonesia, are considered to embody the totality of masculinity and femininity, occupying a spiritual role that bridges the worldly and the divine.
- Sekrata among the Sakalava people of Madagascar are assigned male at birth but raised as girls after displaying feminine behavior in childhood, and they occupy a distinct social role as adults.
The fact that gender-diverse identities emerge across such different societies, with different religions, languages, and value systems, strongly suggests a shared biological basis rather than cultural invention.
How Biology, Psychology, and Environment Interact
The most widely accepted scientific framework for understanding gender identity is the biopsychosocial model, which recognizes that biological predisposition, psychological development, and social environment all play interconnected roles. Biology sets the foundation: genetics and prenatal hormones shape the brain in ways that create a predisposition toward a particular gender identity. Psychological development and social experience then interact with that foundation as a child grows.
Most children develop a stable sense of their gender identity between ages 3 and 5. For transgender children, this is often when the incongruence between their internal sense of self and their assigned sex first becomes apparent. Research suggests the sexually dimorphic brain serves as the anatomical substrate for this development, with gonadal hormones playing a shaping role during both the prenatal period and puberty.
What Gender Dysphoria Is
Not all transgender people experience distress about their bodies, but many do. When that distress is persistent and significant, clinicians call it gender dysphoria. The diagnostic criteria require a marked incongruence between a person’s experienced gender and their assigned sex lasting at least six months, along with clinically significant distress or impairment in daily functioning. This can include a strong desire to be rid of one’s physical sex characteristics, or in adolescents, a desire to prevent the development of those characteristics during puberty.
Gender dysphoria is not a mental illness in the way depression or anxiety is. It describes the distress that arises from the mismatch between body and identity, a mismatch rooted in the biological processes described above. The distinction matters: the identity itself is not a disorder. The suffering comes from the incongruence and, often, from how the world responds to it.
What Happens When Gender Identity Is Affirmed
The clearest evidence that transgender identity is deeply rooted, not a phase or preference, comes from what happens when people receive support. A study of 104 transgender and nonbinary youths aged 13 to 20 found striking results over 12 months. At the start, 56.7% had moderate to severe depression, 50% had moderate to severe anxiety, and 43.3% reported self-harm or suicidal thoughts. Among those who received gender-affirming medical care, the odds of depression dropped by 60%, and the odds of suicidality dropped by 73%, compared to those who had not yet received care.
These outcomes align with what major medical organizations recognize. The World Professional Association for Transgender Health’s Standards of Care, now in its eighth version, is built on the best available science and expert professional consensus. It provides clinical guidance aimed at helping transgender people achieve lasting comfort with their identity while optimizing physical health and psychological well-being. The approach is grounded in decades of evidence showing that when the biological reality of gender identity is acknowledged and supported, people do better.