In the mid-19th century, researchers claimed they could tell the sex of an individual just by looking at their disembodied brain. But a new study finds that human brains do not fit neatly into “male” and “female” categories. Indeed, all of our brains seem to share a patchwork of forms; some that are more common in males, others that are more common in females, and some that are common to both. The findings could change how scientists study the brain and even how society defines gender.
“Nobody has had a way of quantifying this before,” says Lise Eliot, a neuroscientist at Chicago Medical School in Illinois who was not involved in the study. “Everything they’ve done here is new.”
As soon as scientists could image the brain, they began hunting for sex differences. Some modest disparities have been reported: On average, for example, men tend to have a larger amygdala, a region associated with emotion. Such differences are small and highly influenced by the environment, yet they have still been used to paint a binary picture of the human brain, “even when the data reveal much more overlap than difference between males and females,” Eliot says.
So in the new study, researchers led by Daphna Joel, a behavioral neuroscientist at Tel Aviv University in Israel, tried to be as comprehensive as possible. Using existing sets of MRI brain images, they measured the volume of gray matter (the dark, knobby tissue that contains the core of nerve cells) and white matter (the bundles of nerve fibers that transmit signals around the nervous system) in the brains of more than 1400 individuals. They also studied data from diffusion tensor imaging, which shows how tracts of white matter extend throughout the brain, connecting different regions.
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