Using twin data to examine heritable and intrauterine hormonal influences on transgender and gender diverse identities

Abstract

Abstract Previous reports have suggested genetic and prenatal hormonal contributions to gender diversity, albeit using small datasets. To further examine this issue, we recruited 27 twin pairs from two Australian specialist gender clinics, and obtained details of zygosity and self-reported gender identity. To increase the power of our analysis, we pooled our observations with previously published twin data that used comparable methods to ours. The pooled sample of 463 twin pairs comprised 47/222 (21.2%) MZ and 21/241 (8.7%) DZ concordant pairs. Based on a 1% population prevalence estimate, the estimated relative risk ratios for transgender concordance were 21.2 for MZ pairs (95%CI 16.4–27.3), and 8.7 for DZ pairs (95%CI 5.8–13.1). These findings suggest a substantial genetic contribution to gender diversity. In contrast, the combined results for concordance among same and opposite sex DZ pairs were 12/131 (9.2%) and 9/110 (8.2%) respectively, and did not support a role for prenatal sex hormones in the occurrence of gender diversity.