staticnak1983/Getty ImagesPublic debate surrounding transgender and gender diverse (trans) people often centres on one fundamental question: what makes someone trans?Most of us would agree that our gender is not a choice, but a core part of who we are. When our gender aligns with the sex listed on our birth certificate, it’s rarely ever questioned. Instead, it’s treated as a biological reality.However, when it comes to trans people, an alternate explanation is frequently floated: that trans identities and gender diversity are the result of environmental influences, such as social trends or peer pressure.This framing treats a trans identity as a choice that can – and, implicitly, should – be unlearned. Such messaging is increasingly being used to deny the existence of trans people, restrict access to medically necessary health care, and fuel discrimination.Our study of twins challenges this idea and shows the likelihood of being trans is influenced by your genes.The power of twin studiesTwin studies are a powerful tool in research. Identical twins share 100% of their genes, whereas non-identical twins share 50% on average – the same as non-twin siblings.By comparing these two types of twins, we can explore the extent to which genetics and the environment contribute to human traits.If genes are involved, we expect identical twins to share traits (for example, both identifying as trans) more often than non-identical twins, simply due to their higher degree of genetic similarity.Conversely, if genes aren’t involved, we expect to see similar rates of shared traits among identical and non-identical pairs.Our research examined data from 463 twin pairs where at least one was trans – the most comprehensive analysis on this topic to date.After determining whether twin pairs were identical or non-identical, we measured how often both twins were trans, compared to instances where only one was trans.What we foundOur results showed that among identical twins, if one was trans, the likelihood that the other was also trans was 21.2%. For non-identical twins, this rate was significantly lower, at 8.7% – though this was still well above the 1–2% prevalence of trans identities in the general population.Non-twin siblings weren’t included in our research, but we’d expect their rate would be similar to that seen among non-identical twins.If being trans were purely a social phenomenon, we would not see this step-wise difference where the chance of being trans goes up with increasing genetic similarity.Our data thus provide strong evidence that genes are involved.More than just genesIt’s also important to highlight another finding in our data – that most twin pairs, even among those who are identical, are not both trans.This comes as no surprise to researchers who study twins, who have documented similar patterns for many other human traits. Almost all human traits, such as personality characteristics, height, and sexuality, are shaped by both genetic and environmental factors.It’s likely that gender identity is the same.Interplay of genetics and the environmentGenes and the environment do not act independently, and the influence of genetics on any complex human trait does not stem from variations in a single gene. Rather, hundreds or even thousands of genes work alongside and react to environmental influences to form an enormous range of human characteristics. Environmental factors can affect the activity of our genes throughout the course of our lives. Such environmental influences encompass the full breadth of human experiences, all the way back to conception. Both identical and non-identical twins share environmental experiences, especially within the womb and as children raised together in the same family. But each twin also has unique experiences that contribute to their individual differences. So, it really is a complex interplay between genes and the environment.Why this matters for public understandingMany anti-trans arguments rely on a belief that gender diversity is entirely socially determined. This myth is frequently weaponised to frame trans identities as optional or reversible. Too often, this becomes a justification for hostile public attitudes and for stripping basic legal protections from trans people.The belief that being trans is purely a social phenomenon also provides a rationale for harmful identity change and suppression practices (also known as conversion therapy).Every major Australian and United States medical association has condemned these practices. However, a recent US Supreme Court case and other national and state policy developments in the US in particular are increasingly providing cover for those performing these harmful practices.A complex and enduring human traitOur findings provide a crucial correction to this harmful public discourse. They confirm that genes and their related biological pathways play a role in shaping gender identity.Fears of trans identities being a social trend or response to peer pressure – and related shifts in policies to restrict trans people’s basic rights – are at odds with the scientific evidence.Gender diversity has existed across millennia and is an inherent part of human diversity. It’s time law and society caught up. Read more: Critics claim gender clinics are seeing an excess of trans boys. But new data suggest otherwise Ken Pang receives research funding from the Australian National Health and Medical Research Council (NHMRC) and the Medical Research Future Fund. He is a member of the Australian Professional Association for Trans Health, the World Professional Association for Transgender Health, and the editorial board of the journal, Transgender Health. He is also a member of the NHMRC Gender Guidelines Development Committee, and the views expressed in this article do not necessarily reflect those of the committee.Anja Ravine is a member of the Australian Professional Association for Trans Health and the Human Genetics Society of Australasia. Kellan Baker serves as the Senior Advisor for Health Policy at the Movement Advancement Project (MAP), an independent, nonprofit think tank in the United States. He has previously received research funding from the US National Human Genome Research Institute and is a member of the World Professional Association for Transgender Health and the US Professional Association for Transgender Health. Will Conabere does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.