A study published in Scientific Reports presents evidence that transgender women – individuals assigned male sex at birth who experience gender dysphoria and identify as female – have differing brain cell volume in an area of the cortex that mediates self-awareness, body image, and emotion.
For their investigation, University of São Paulo Medical School researchers used advanced MRI scans to quantify the amount and distribution of gray matter in 20 cisgender men (CM), 20 cisgender women (CW), 20 transgender women who had never taken hormone therapy (TNTW), and 20 transgender women who had taken at least one year of hormone therapy (TTW).
After analyzing the entire brain of each participant, the Brazilian team found that both groups of transgender women showed lower gray matter volume in the insular lobes (there is one in each hemisphere) compared with cisgender women. Though the data is tentative, previous studies have indicated that people with gender dysphoria display irregular connectivity between insular neurons and the rest of the brain.
“These alterations in the insula could be related to the neural network of body perception and may reflect the distress that accompanies gender dysphoria,” the authors write.
Curiously, the only other significant difference was that treated, but not untreated transgender women, had lower left insula volume compared with cisgender males. The authors are unable to explain at this point why individuals who have not taken feminizing hormones, and are therefore more likely to experience gender dysphoria than their TTW peers, don't also show insular volumes differing from CM.
That puzzle aside, the findings support the theory that the formation of brain structures during early development can impact an individual’s sexual identity later in life, regardless of their anatomical features. And although previous MRI studies have found contrasting differences specifically in the insular lobes of transgender people verse cisgender people, variation in areas related to body perception seems to be a pattern.
"The evidence is building up that it's not a matter of ideology,” said lead author Carmita Abdo in a statement.
Yet despite well-established differences between the brains of males and females, Abdo and her colleagues stress that the brain patterns of TW observed in their investigation, and others, are not reflective of a male having “female brain characteristics”.
Boiling transgender identity down to masculine and feminine brain types is shortsighted, according to first author Giancarlo Spizzirri, because "there's no such thing as a typically female or male brain. There are slight structural differences, which are far more subtle than the difference in genitals, for example,” he said. “Brain structures vary greatly among individuals."
Next, the group hopes to examine brain scans from children with gender dysphoria in order to determine how early noticeable differences in structure may be detected.