Some children insist, from the moment they can speak, that they are not the gender indicated by their biological sex. So where does this knowledge reside? And is it possible to discern a genetic or anatomical basis for transgender identity? Exploration of these questions is relatively new, but there is a bit of evidence for a genetic basis. Identical twins are somewhat more likely than fraternal twins to both be trans.

Male and female brains are, on average, slightly different in structure, although there is tremendous individual variability. Several studies have looked for signs that transgender people have brains more similar to their experienced gender. Spanish investigators—led by psychobiologist Antonio Guillamon of the National Distance Education University in Madrid and neuropsychologist Carme Junqué Plaja of the University of Barcelona—used MRI to examine the brains of 24 female-to-males and 18 male-to-females—both before and after treatment with cross-sex hormones. Their results, published in 2013, showed that even before treatment the brain structures of the trans people were more similar in some respects to the brains of their experienced gender than those of their natal gender. For example, the female-to-male subjects had relatively thin subcortical areas (these areas tend to be thinner in men than in women). Male-to-female subjects tended to have thinner cortical regions in the right hemisphere, which is characteristic of a female brain. (Such differences became more pronounced after treatment.)

“Trans people have brains that are different from males and females, a unique kind of brain,” Guillamon says. “It is simplistic to say that a female-to-male transgender person is a female trapped in a male body. It's not because they have a male brain but a transsexual brain.” Of course, behavior and experience shape brain anatomy, so it is impossible to say if these subtle differences are inborn.

Other investigators have looked at sex differences through brain functioning. In a study published in 2014, psychologist Sarah M. Burke of VU University Medical Center in Amsterdam and biologist Julie Bakker of the Netherlands Institute for Neuroscience used functional MRI to examine how 39 prepubertal and 41 adolescent boys and girls with gender dysphoria responded to androstadienone, an odorous steroid with pheromonelike properties that is known to cause a different response in the hypothalamus of men versus women. They found that the adolescent boys and girls with gender dysphoria responded much like peers of their experienced gender. The results were less clear with the prepubertal children.

This kind of study is important, says Baudewijntje Kreukels, an expert on gender dysphoria at VU University Medical Center, “because sex differences in responding to odors cannot be influenced by training or environment.” The same can be said of another 2014 experiment by Burke and her colleagues. They measured the responses of boys and girls with gender dysphoria to echolike sounds produced by the inner ear in response to a clicking noise. Boys with gender dysphoria responded more like typical females, who have a stronger response to these sounds. But girls with gender dysphoria also responded like typical females.

Overall the weight of these studies and others points strongly toward a biological basis for gender dysphoria. But given the variety of transgender people and the variation in the brains of men and women generally, it will be a long time, if ever, before a doctor can do a brain scan on a child and say, “Yes, this child is trans.”

>>Read more about transgender kids and what it takes to help them thrive.

The Author

FRANCINE RUSSO is a veteran journalist, specializing in psychology and behavior. She is also a speaker and author of They're Your Parents, Too! How Siblings Can Survive Their Parents Aging without Driving Each Other Crazy.