<p>Amidst growing visibility of gender diversity, the aetiology of gender identity has become a subject of increasing public interest. Prompted by the growing public debate, we review here the extant twin literature regarding the origins of gender diversity. Literature was reviewed systematically, searching Medline, Embase and PubMed databases. Studies were deemed eligible if they: (i) conducted a twin study, (ii) investigated gender identity or gendered behaviour, and (iii) provided an estimate of the magnitude of genetic or environmental contribution. After screening 290 non-duplicate titles and abstracts, 16 articles were included in the final review. Most eligible studies provided evidence of both genetic and environmental contributions to gender identity and gendered behaviour. For gendered behaviour, genetic contributions ranged from 0.10 to 0.77, non-shared environmental contributions ranged from 0.15 to 0.75, and shared environmental contributions ranged from 0.00 to 0.49. For gender identity, genetic contributions ranged from 0.00 to 0.84, non-shared environmental contributions ranged from 0.15 to 0.96 and shared environmental contributions ranged from 0.00 to 0.70. Given the variability in results and methodology between studies, the true magnitude of these contributions remains unclear. No consistent differences in contributions were identified between assigned males and assigned females. While there is also recent evidence that prenatal hormone exposure may contribute to gender identity, the overall evidence from the literature is inconsistent. Twin studies indicate both genetic and environmental contributions to gender diversity. These results are important to inform ongoing public debate in this area and highlight the complex interplay of both genetics and environment.</p>