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Genetics and gender
Genetic research into gender identity and gender dysphoria is sometimes invoked as proof that trans identification is innate and immutable. A sober look at the evidence shows something else: there is a modest hereditary component, no conclusive proof of genetic determination, and biological sex itself is well determined by chromosomes, gametes and anatomy.
Biological sex is binary and stable
In humans — as in all mammals — sex consists of two categories, defined by the type of gamete the body is organised to produce: sperm cells (male) or egg cells (female). This is not a social convention but a fundamental biological fact, determined by chromosomal configuration (XY versus XX), gonadal development and anatomy. Sex is observed at birth, not "assigned". The rare intersex variations (together about 0.02% of the population by narrow definitions) are developmental disorders within the binary system, not a third sex.
Twin research: modest hereditary component
Classical twin research compares identical with fraternal twins. The concordance for gender dysphoria is higher in identical twins than in fraternal twins, which indicates a hereditary component. But the concordance is far from 100% — often around 30–40% in identical twins. That means: even with identical genes it is quite possible that one child develops dysphoria and the other does not. Environment, social context and individual developmental factors thus play a leading role alongside genetics. Moreover, the published twin studies are small and methodologically limited.
Candidate genes and GWAS: inconsistent
Decades of research into specific genes — especially those involved in hormone receptors — have not produced a single gene with a strong, reliably replicated influence on gender identity. Genome-wide association studies (GWAS), including a large analysis in 2023, found some suggestive associations but no genome-wide significant hits. This fits with what we know from other complex psychological traits: many genes with minuscule effects, strongly intertwined with environmental factors.
Prenatal hormones: influence on behaviour, not on identity
Prenatal exposure to androgens influences gender-related behaviour, as investigated in girls with congenital adrenal hyperplasia (CAH). But this effect is predominantly on play and interest patterns, not on a specific "gender identity": the great majority of CAH girls do not develop a trans identity. The step from "prenatal hormones influence behaviour" to "gender identity is biologically determined" cannot be made scientifically.
The demographic puzzle
A strong genetic theory would also need to explain why the prevalence of trans identification in Western countries has multiplied tenfold to twentyfold since 2010, with a striking shift from predominantly biological boys to predominantly adolescent girls. The human genome does not change on this time scale. An explanation in terms of socio-cultural factors and peer contagion — see ROGD — is more plausible than an unexplained "epigenetic" reversal.
Intersex is not a third sex
In public communication the existence of intersex conditions is sometimes invoked as proof that sex is "a spectrum". This is scientifically incorrect. Intersex conditions are developmental variations within the binary sex system (XY with androgen insensitivity, XX with CAH, etc.). The total prevalence with strict definitions is about 0.02%, not the regularly cited 1.7% (which embraces broad definitions such as late puberty). Sex is binary; intersex means a developmental disorder, not an "intermediate form".