A new twin study, the most comprehensive to date, shows that identical twins are more than twice as likely as non-identical twins to both be trans, providing strong evidence that genes help shape gender identity. The finding directly contradicts claims that trans identities are a social trend or a choice. Gender diversity is a complex human trait, shaped by both genetics and the environment, and it has existed across recorded history.

The argument we keep having to answer

Trans people should not have to prove their existence through a genetics paper. I want to say that first, plainly, because everything that follows could otherwise be read as though we have accepted the terms of the debate on offer. We have not. A trans woman does not need a gene for her identity to be real, any more than a cisgender woman does. Identity is not validated by biology, and biology is not validated by what makes critics comfortable.

And yet. When research emerges that meets the "it's just a social trend" argument on its own ground and dismantles it with data, that is worth paying attention to. Not because trans people needed the permission, but because the argument has consequences. It has been used to justify conversion therapy. It has been used to strip healthcare from young people. It has been used to frame a community as a contagion. So when the science speaks clearly, I think we should hear it.

What the research found

The study, published in The Conversation and drawing on data from 463 twin pairs where at least one twin was trans, is the most comprehensive analysis of its kind. The method is elegant in its simplicity. Identical twins share 100% of their genes. Non-identical twins share around 50%, the same as any sibling. If genes are involved in a trait, you expect identical twins to share it more often. If genes play no role at all, the rates should look roughly similar across both groups.

What the researchers found was a clear step-wise pattern. Among identical twins, if one twin was trans, there was a 21.2% chance the other was too. Among non-identical twins, that figure dropped to 8.7%. Both figures are substantially higher than the 1 to 2% prevalence of trans identities in the general population, which itself tells you something. But it is the gap between the two groups that matters: that gap is exactly what you would expect if genetics is part of the picture.

The researchers are careful and honest about what this does and does not show. Most identical twins in the study were not both trans, and they are not surprised by that. Virtually every complex human trait, from personality to height to sexuality, is shaped by both genes and environment, and the relationship between them is not simple or linear. Environmental factors can influence how genes are expressed throughout a lifetime, all the way back to the womb. Gender identity, the evidence suggests, works the same way.

Why the "social contagion" argument was always wrong

The claim that trans identities are produced by peer pressure or social trends has always had a logical problem: it requires you to believe that a person would choose an identity that brings them discrimination, gatekeeping, hostility, and in many parts of the world genuine danger, simply because they saw someone online do it. That has never been a serious account of human motivation.

But the twin data adds something more specific. If being trans were purely a social phenomenon, you would not see the difference between identical and non-identical twins. Social environments, family, school, the internet, are shared by both types of twins in broadly similar ways. The thing that differs is their degree of genetic similarity. And it is precisely along that line that the rates diverge. The "social contagion" story cannot account for that pattern. The genetics story can.

The researchers put it plainly: gender diversity has existed across millennia, and it is an inherent part of human diversity. That is not a new claim; anthropologists, historians, and trans communities themselves have been saying it for decades. What is new is a dataset large enough to make the genetic contribution visible and measurable.

What this means for the arguments being used against trans people

The conversion therapy industry, and yes, it is an industry, rests on the premise that trans and gender diverse identities can and should be changed. Every major Australian and American medical association has condemned these practices. And yet, as the researchers note, recent legal and policy developments in the United States have provided cover for those who continue to perform them.

That cover becomes harder to sustain in the face of evidence like this. You cannot credibly argue that an identity rooted in genetic pathways, shaped across a lifetime by a complex interaction of biology and experience, can be corrected by a practitioner with an ideological agenda and a conversion programme. The science does not support it. The ethics have never supported it. What is left is the ideology, stripped of its pretence.

The same is true of policies that restrict healthcare on the grounds that trans identities are a phase, a trend, or a product of confusion. If gender identity has a genetic component, the question shifts from whether trans people are real to whether we are prepared to treat them with the same care and seriousness we extend to everyone else. That is the question I would like to see debated.

One thing I want to hold onto

The finding that most identical twin pairs were not both trans is not a weakness in this research. It is a description of how human beings actually work. We are not the output of our genes alone. We are shaped by everything that happens to us, by the families we grow up in, the bodies we find ourselves in, the cultures we move through, the relationships that form us. That complexity is not a reason to doubt trans identities. It is a reason to take seriously the full humanity of the people who hold them.

Trans people do not need a genetic explanation to deserve respect, healthcare, and legal protection. But when the evidence is this clear, and the arguments against them are this threadbare, it is worth saying so.