Genetics and Gender
Genetic research into gender identity and gender dysphoria is sometimes presented as evidence that trans-identification is innate and immutable. A sober look at the evidence reveals something else: there is a modest heritable component, no conclusive evidence of genetic determinism, and biological sex itself is clearly 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 organized to produce: sperm (male) or eggs (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 (approximately 0.02% of the population in total according to narrow definitions) are developmental disorders within the binary system, not a third sex.
Twin studies: a modest heritable component
Classical twin studies compare identical twins with fraternal twins. Concordance for gender dysphoria is higher among identical twins than among fraternal twins, indicating a heritable component. But concordance is far from 100%—often around 30 to 40% among identical twins. This means that even with identical genes, it is entirely possible for one child to develop dysphoria while the other does not. Environment, social context, and individual developmental factors therefore play a major role alongside genetics. Moreover, the published twin studies are small and methodologically limited.
Candidate genes and GWAS: inconsistent
Decades of research into specific genes—particularly those involved in hormone receptors—have not identified a single gene with a strong, reliably replicated influence on gender identity. Genome-wide association studies (GWAS), including a large analysis in 2023, found several suggestive associations but no genome-wide significant hits. This is consistent with what we know about other complex psychological traits: many genes with minuscule effects, deeply intertwined with environmental factors. A more detailed discussion of GWAS research can be found at genderid.nl.
Prenatal hormones: influence on behavior, not identity
Prenatal exposure to androgens influences gender-related behavior, as studied in girls with congenital adrenal hyperplasia (CAH). But this effect is primarily seen in play and interest patterns, not in a specific “gender identity”: the vast majority of girls with CAH do not develop a trans identity. The leap from “prenatal hormones influence behavior” to “gender identity is biologically determined” cannot be made scientifically.
The demographic puzzle
A strong genetic theory would also have to explain why the prevalence of trans-identification in Western countries has increased ten- to twentyfold since 2010, with a striking shift from predominantly biological boys to predominantly adolescent girls. The human genome does not change on this timescale. An explanation based on sociocultural factors and peer contagion—see ROGD—is more plausible than an unexplained “epigenetic” shift.
Intersex is not a third sex
In public communication, the existence of intersex conditions is sometimes cited as evidence 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 under strict definitions is approximately 0.02%, not the frequently cited 1.7% (which includes broad definitions such as late-onset puberty). Sex is binary; intersex means a developmental disorder, not an “intermediate form.”