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No genetic marker for gender identity

No genetic marker for gender identity has been established so far. According to the Cass Review, there is no clear evidence of a simple biological cause of gender incongruence, although a biological predisposition may play a role in some people (para. 8.21) (source).

What GWAS does — and what it does not find here

Genome-wide association studies (GWAS) compare hundreds of thousands of SNPs between groups to find associations with a phenotype. In this way small but reproducible polygenic signals have been found for homosexuality, schizophrenia and autism. For 'gender identity', no genetic marker has been established so far; the available research is described below. This bears directly on the metaphysical claim on which the whole diagnosis rests.

What the studies really show

Heylens et al. (2012) compiled the published twin cases: of 23 identical twin pairs, nine (39.1%) were concordant for gender identity disorder, against none of 21 same-sex fraternal twin pairs; the authors see this as a possible role for genetic factors (source). Polderman et al. (2018) hypothesise that gender identity is a complex, multifactorial trait with a heritable polygenic component (source). Theisen et al. (2019) carried out exome sequencing in 30 transgender people (13 trans men and 17 trans women) and found 21 rare variants in 19 genes linked to sexually dimorphic brain development; the authors see this as a starting point for further research, not as a marker (source). These are indications and hypotheses, not an established genetic marker.

Twin studies: limited indications

Twin studies are limited. According to the Cass Review, most relied mainly on parental report, did not follow participants into adulthood and/or had small samples (para. 8.13); older studies suggested that identical twins more often shared the same gender identity than fraternal twins, pointing to some genetic influence. A more recent Swedish register study (Karamanis et al., 2022; 2001-2016) found no evidence of genetic influence among those diagnosed, but did find that opposite-sex fraternal twins were far more often concordant than ordinary siblings (37% versus 0.16%) (para. 8.14) (source).

Why this matters

Without genetic evidence, the claim 'born this way' remains an unproven hypothesis. The Cass Review notes that, for some clinicians, the search for a biological cause strengthens the justification for medical treatment (source). The same gap appears with the alleged brain marker and the wider absence of any measurable marker . In the absence of a marker, assessment relies on self-report within a clinical assessment; according to the Cass Review, few guidelines recommend formal measurement tools and the existing ones are poorly validated (para. 10.8) (source). The Cass Review called the evidence for this practice 'remarkably weak'. In her foreword, Hilary Cass writes that there are few other areas of healthcare where professionals are so afraid to openly discuss their views (source).