A new, data-oriented review on the evidence for biological contributions to gender identity was published by the International Gender Diversity Genomics Consortium on the Behavior Genetics journal.

After recalling the failure of previous theories of gender as pure socialization, and the failure of behavioral modification techniques etc., the authors review 11 high-quality studies out of a pool of 302 candidates. The results support what’s called a polygenic threshold model. In such a model, multiple genes (and not a single one) contribute significantly to, but do not determine, a complex trait. The result is a wide distribution of trait values, caused by multiple genetic factors interacting in complex ways with multiple environmental factors. This kind of model was highly successful in improving our understanding of other complex heritable traits, like autism; their apparent good fit to gender identity suggests that, like autism, gender is distributed over a spectrum, rather than divided sharply in a binary line; and that it’s rooted in biology/genetics, but not completely determined by it. [I note that this is broadly the same conclusion reached by both camps of the debate: those scientists who come from a “biological” bias, like Milton Diamond, and those scientists who come from a “social construction” bias, like Anne Fausto-Sterling, both seem to eventually infer from the evidence some sort of nature-nurture interaction, such as Diamond’s organization-activation theory or Fausto-Sterling’s developmental dynamics.]

To minimize cultural biases and avoid crude scores based on cultural stereotypes, the studies under review had their results rated using Lippa’s technique of gender diagnosticity: the Bayesian probability that an individual is male or female, according to discriminant analysis of a broad set of indicators (those who optimally distinguish membership among two groups).

Adult studies showed that:

Gender identity of identical twins is conformant at much higher rates than the general population, and (in multiple studies) at about double the rate than that of non-identical twins (a distribution considered to be a distinguishing mark of genetic factors);

Measures of genetic influence on (heritability of) gender diagnosticity range from 13% to 50% in different studies—which is consistent with other human traits known to be heritable;

Influence of shared environment (culture, household) had negligible effect, suggesting that the non-genetic factors are unique influences [measurement errors? hormonal exposure? unpredictablee dynamic-system effects?];

The ratios are about the same for women and men.

Child and adolescent studies are more conflicting, with some studies (but not others) showing a significant shared environment component especially for children assigned male at birth (and, conversely, assigned female children reaching as far as 84% heritability in one study). These complex results may come from the variable timing of gender identity onset, or just because there’s less data available and less statistical power. What the data do make clear, however, is that the genetic factors exist and are significant, in children as in adults.

Given that complex, partially heritable traits (such as gender identity) are normally influenced by many genes, each with small effect sizes, the authors urge a genome-wide association study with a large enough sample size to better identify genetic factors.