That’s exciting news for anyone who studies the brain or gender. And it’s a step toward validating the experiences of those who live outside the gender binary.
“Whereas a categorical difference in the genitals has always been acknowledged, the question of how far these categories extend into human biology is still not resolved,” the authors wrote in the study. Structural differences in the brain — and differences in behavior — are often taken as evidence that brains can be distinctly male or female. For this to be true, the authors note, the differences would have to be consistent: Those who were biologically male would almost always have to have “male” features and not “female” ones in their brain.
But in analyzing the MRI exams of about 1,400 individuals, researchers led by Daphna Joel of Tel Aviv University found that mash-ups were more common. The researchers said they think their study is the first to look for brain differences between genders by using the brain as a whole instead of pointing out individual structures and features, including size and the amount of gray vs. white matter in isolation.
“Nobody has had a way of quantifying this before,” Lise Eliot, a neuroscientist at the Rosalind Franklin University of Medicine and Sciences’ Chicago Medical School in Illinois who was not involved in the study, told Science Magazine. “Everything they’ve done here is new.”
The authors found that only a very small number of the brains studied had features that were entirely male, female or intermediate between the two.
Brains “with features that are consistently at one end of the “maleness-femaleness” continuum are rare,” they said. “Rather, most brains are comprised of unique ‘mosaics’ of features, some more common in females compared with males, some more common in males compared with females, and some common in both females and males.”
Rockefeller University’s Bruce McEwen, who edited the study manuscript but did not participate in the research, told New Scientist magazine that the findings would probably surprise some. “We are beginning to realize the complexity of what we have traditionally understood to be ‘male’ and ‘female,’ and this study is the first step in that direction,” he said. “I think it will change peoples’ minds.”
Joel said that she hopes the study will help do away with assumptions about gender differences. “We separate girls and boys, men and women all the time,” she told New Scientist. “It’s wrong, not just politically, but scientifically — everyone is different.”