Gender identity is shaped by both nature and nurture, though the biological ingredients appear to carry more weight. Twin studies, brain imaging, and prenatal hormone research all point to a strong biological foundation, while social environment, family support, and cultural context influence how that identity is expressed and experienced over time.
What Twin Studies Reveal
Twin research offers one of the clearest windows into the nature-nurture question because identical twins share 100% of their DNA while fraternal twins share about 50%. If gender identity were purely social, identical and fraternal twins would show similar rates of both being transgender. If genetics played a role, identical twins would match more often.
That’s exactly what researchers find. A pooled analysis of 463 twin pairs found that identical twins were both transgender 21.2% of the time, compared to 8.7% for fraternal twins. That 2.6-fold difference was statistically significant and strongly suggests genetic involvement. Both rates are dramatically higher than the roughly 1% prevalence of transgender identity in the general population, which means familial factors (shared genes and shared prenatal environment) matter considerably. But the fact that identical twins don’t match 100% of the time tells us genetics aren’t the whole story. Something beyond DNA is also at work.
Hormones in the Womb
One of the strongest biological candidates is prenatal hormone exposure. During fetal development, sex hormones shape not just reproductive anatomy but also the developing brain. These are called “organizational effects” because they happen during a sensitive window and are thought to be permanent.
Much of what we know comes from studying people with congenital adrenal hyperplasia (CAH), a condition where the adrenal glands produce unusually high levels of androgens before birth. Girls with CAH are exposed to elevated androgens in utero, and their brain function often shifts in a male-typical direction. In one study, girls with CAH showed greater activation in the amygdala (a brain region involved in processing emotions) when viewing fearful and angry faces, at levels statistically indistinguishable from typical boys. Other research found differences in how their brains handled emotional memory, with patterns again resembling those of boys rather than girls.
These findings suggest that the hormonal environment a fetus develops in can permanently influence brain organization in ways that relate to gendered behavior and, potentially, identity. The neural pathways affected are still being mapped, and researchers acknowledge that existing studies are limited by small samples and the difficulty of isolating hormone effects from other variables. Still, the evidence consistently points toward prenatal androgens as one piece of the puzzle.
Brain Differences Tied to Identity
Neuroimaging research has found that the brains of transgender individuals often show structural features that align more closely with their experienced gender than with their sex assigned at birth. Studies have identified shifts in multiple brain regions, including a small cluster of cells called the bed nucleus of the stria terminalis, as well as areas involved in body perception and self-awareness like the insula, the region around the central sulcus, and the posterior cingulate. One review described the pattern in transgender women as the brain being “shifted towards gender identity,” meaning their brain anatomy fell between typical male and female patterns, leaning toward female.
These differences appear to exist before any hormone therapy, which suggests they aren’t simply a result of treatment. They point to gender identity having a neurological basis that develops early, likely through the combined influence of genetics and prenatal hormones.
When Gender Identity Emerges
Children begin showing awareness of gender remarkably early. By age 2, most can distinguish between boys and girls. By 3, most identify themselves as one or the other, and that label may or may not match their sex assigned at birth. Between ages 3 and 5, some children explore gender fluidly, sometimes identifying differently from one day to the next. This is considered a normal part of development.
By age 6 or 7, many children feel more settled in their gender identity and spend less energy broadcasting it to others. From age 8 onward, most continue with a gender identity that matches their birth assignment, but some preteens and teens realize during puberty that their internal sense of gender differs from what they were assigned. The fact that gender identity shows up so early in life, often before children have any concept of social expectations around gender, is frequently cited as evidence for a biological component.
The David Reimer Case
One of the most influential cases in this debate involved David Reimer, a boy who lost his penis in a botched circumcision as an infant in the 1960s. Following the advice of psychologist John Money, who believed children are psychosexually neutral at birth and can be raised as either gender, Reimer’s parents consented to surgery that removed his testes and constructed female-appearing genitalia. He was raised as a girl.
Money published the case as a success, and it became a cornerstone of the theory that gender identity is entirely learned. But that conclusion was wrong. Reimer never accepted his female assignment. He displayed male-typical behavior throughout childhood, and at age 14, after threatening suicide, his parents told him the truth. He immediately began living as male and underwent surgeries to restore his body. Reimer took his own life in 2004 at age 38. His case is now widely understood as powerful evidence that socialization alone cannot override a person’s innate gender identity.
How Social Environment Shapes Expression
Biology lays the groundwork, but social context clearly matters for how gender identity develops and whether a person feels safe expressing it. Research on socially transitioned transgender youth (children whose families supported them in living as their identified gender) found that these children persisted in their transgender identities at much higher rates than older clinical studies had reported. The researchers attributed this partly to family support and partly to growing up in an era with greater visibility and acceptance of transgender people. Children who feel less shame and less pressure to suppress their identity are more likely to maintain it openly.
This doesn’t mean social support “causes” someone to be transgender. Rather, it suggests that environment determines whether an existing identity can be expressed, explored, and stabilized, or whether it gets buried under social pressure. The American Psychological Association’s current position reflects this complexity: there is no single explanation for why some people are transgender, and biological factors like genetics and prenatal hormones, early life experiences, and later social experiences may all contribute.
The Biopsychosocial Picture
The most accurate framework for understanding gender identity is a biopsychosocial one, where biology, psychology, and social environment all interact. Genetics set a range of possibilities. Prenatal hormones organize the brain in ways that predispose someone toward a particular gender identity. A person’s psychological development, including their growing self-awareness, refines that identity. And the social world around them, from family to culture, shapes how that identity is understood, labeled, and lived.
What the evidence does not support is the idea that gender identity is a simple choice or that it can be reliably overridden by how a child is raised. The twin data, the brain research, the prenatal hormone findings, and cases like David Reimer’s all converge on the same point: biology provides a strong foundation that social forces can influence but rarely reverse. The “nature or nurture” framing is too simple. Gender identity is nature and nurture, with nature doing more of the heavy lifting than many people once assumed.