Gender dysphoria could be triggered by childhood stress and potentially cured with medicine, a scientist has claimed.
Dysphoria is when someone feels their gender identity and their biological sex don't match and it may lead to them choosing to be transsexual.
Dr Stephen Gliske, a neurologist at the University of Michigan, disputes the widely-held theory dysphoria is caused by differences in physical brain sizes.
Instead, he claims the medically-recognised condition is caused by changes in brain activity which happen during someone's life.
And this, he said, means they could be 'resolved' without the person feeling the need to change sex or live as another gender – although he did not suggest how.
Critics have hit back at Dr Gliske's theory, branding it 'problematic' because it suggests dysphoria is something to be fixed like an illness.
A University of Michigan neurologist claimed gender dysphoria could be caused by changes in the activity in people's brains, rather than them developing in a different way (stock image of nerves in the brain)
One argued other people's attitudes were where change was needed, rather than the brains of people with gender dysphoria.
NHS treatment currently revolves around giving people psychotherapy or, in some cases, transition treatments like hormones or reassignment surgery.
But Dr Gliske said the current understanding of gender dysphoria is too 'inaccurate' to justify giving someone sex-change surgery.
Dr Gliske reviewed past papers published about dysphoria and published his theory in the medical journal eNeuro.
He said that, in the past, scientists have looked to size variations in areas of the brain to explain why someone may feel they have been assigned the wrong gender.
'The bed nucleus of the stria terminalis (BNST) was found to have a smaller average size in male-to-female (MtF) transgender individuals, with a size more similar to that of an average cisgender female than cisgender male,' he wrote.
Dr Gliske said brain imaging studies don't support the theory that gender dysphoria is caused by differences in the physical shapes of people's brains.
He wrote: '[My] theory proposes that gender dysphoria is not merely due to static changes in anatomy, as in the previous opposite brain sex theory, but instead includes dynamic activity on interacting, functional networks.
Gender dysphoria is a condition in which someone becomes distressed because they don't feel that their biological sex matches the gender they identify as.
For example, someone may feel like a woman and want to live as a woman, but have been born with the anatomy of a man.
Gender dysphoria is a 'recognised medical condition, for which treatment is sometimes appropriate. It is not a mental illness,' according to the NHS.
People who live as a gender which is not the same as their biological sex are called transgender.
Some people may choose to have hormone therapy – for example, to make them grow hair or develop breasts – or to have reassignment surgery to give them the genitals of a person of the sex they identify as.
People diagnosed with gender dysphoria are allowed to legally change