“You can’t look at an individual brain and know if it is male or female,” says Lise Eliot, associate professor of neuroscience at Rosalind Franklin University in Chicago and the author of Pink Brain, Blue Brain. The same applies to psychological traits, such as mathematical ability or empathy. “It’s just not the case that men lack empathy and women are all universally supersensitive,” adds Eliot. “There are statistical differences, but not predictive individual differences. There is tremendous overlap.”

The brains of adult transgender people fit even less neatly into male or female categories. “It is not a matter of the size, it is a matter of the pattern - or how the brain has been built,” says Antonio Guillamon at the National Distance Education University (Uned) in Madrid, who has been involved in some of these studies. At the whole brain level, women who identify as men tend to have female-sized brains, and men who identify as women tend to have male-sized brains, but when Guillamon and his colleagues scanned the brains of men, women and transgender individuals, they found subtle differences in four regions of white matter – fatty tissue harbouring the long spindly projections of nerve cells that connect different brain areas together – between each of these groups.

In women who identified as men, these regions more closely resembled those of male controls, whereas in male-to-female transsexual people, the structure of these regions was halfway between that of control men and women. “The brains of male to female transsexuals are not male brains exactly, and the brains of female to male transsexual are not exactly female,” says Guillamon. However, he cautions that it’s still too early to say whether these differences explain why people feel male or female.

The Lady Gaga hypothesis

Yet other studies have found similar reversals in male and female-typical patterns among transsexuals in small areas of grey matter. One of them is a tiny area of the hypothalamus called INAH3; the animal equivalent of which seems to influence sexual behaviour in rats.

Some claim such findings as evidence that transsexuality has a physical basis rather than being a choice, but not everyone is convinced. One issue is the brain’s innate plasticity, or its ability to rewire itself in response to experience.

“There may be hints of brain differences in transgender people, but you’d expect that because their life experience is going to have been quite different,” says Eliot. “How long they have identified as the other gender; the way they talk; who they played with as children; what sort of jobs they’re involved in – all of these things could affect those same pathways in subtle ways. There is certainly no proof of the Lady Gaga hypothesis that I was ‘Born This Way’.”

That’s not to take anything away from individuals’ profound sense of having the wrong brain for their body, she emphasises. But when it comes to identifying the biological underpinnings of gender identity, possibly the adult brain isn’t the best place to look.