Boy brains and girl brains? Scientists explore sex, gender, and distinct brain networks

In science, the term “biological sex” encompasses a variety of genetic, hormonal, and anatomical characteristics. People are typically assigned “male” or “female” as their sex at birth, although the medical establishment in recent years has begun to acknowledge that sex doesn’t always fall neatly into binary categories. 

Gender, by contrast, has more to do with a person’s attitudes, feelings, and behavior—and may not always align with the sex they were assigned at birth.

New research has looked at brain imaging data from nearly 5000 children to reveal that gender and sex aren’t just distinct from one another in society—they also play unique roles in biology.

Sex certainly seems to affect a person’s risk of developing various psychiatric and other brain-related conditions—but scientists aren’t entirely sure why. 

Attention-deficit/hyperactivity disorder for example, is more commonly diagnosed in individuals who were assigned male at birth (AMAB), whereas those assigned female at birth (AFAB) are more likely to exhibit symptoms of anxiety. 

It’s unclear, however, whether these differences are actually driven by sex, or have more to do with how people are perceived and treated based on their sex or gender.

Both sex and gender are important to study because they’re “essential components of identity,” lead author and neuroscientist Elvisha Dhamala said. 

“It’s becoming more and more clear that just looking at sex itself is not enough,” she added. “It’s not going to give us all the answers.”

To disentangle the effects of sex on brain activity from those of gender, Ms Dhamala and her colleagues analyzed brain imaging data collected as part of the Adolescent Brain Cognitive Development (ABCD) Study—the largest long-term study of brain development and child health in the U.S. 

The team looked at functional magnetic resonance imaging (fMRI) scans from 4727 children ages 9 and 10 years, all of whom were either assigned female (2315 children) or male (2442) at birth.

Collecting gender data was a little more complicated. The ABCD Study takes a nuanced approach by asking participants four questions centered on gender, including whether they feel satisfied with the gender they were assigned at birth, researchers explained.

Because 9- and 10-year-olds may not have a completely developed sense of their own gender identity, the ABCD Study also asks participants’ parents a larger set of questions about certain behaviours in their child and signs of gender dysphoria. 

For example, whether they’ve noticed that their AMAB child imitates female characters on TV, or that their AFAB child consistently expresses a wish to be a boy or a man.

During fMRI scanning, ABCD Study participants were asked to do a set of neurocognitive tests—related to things like memory and emotional processing—to see how different brain regions and networks communicate with one another. Researchers then fed those scans to machine learning algorithms to determine whether these patterns of activity in a child’s brain could be used to identify their sex and gender.

Although the algorithms accurately predicted a participant’s assigned sex at birth, they had a harder time determining gender. The models couldn’t predict self-reported gender for either sex, but did identify some brain activity patterns that were consistently associated with parent-reported gender—possibly because this latter measurement captured multiple dimensions of gender identity.

These gender-associated brain networks were distinct from those associated with assigned sex at birth. Networks that showed patterns of connectivity associated with sex included those that play a role in sensory processing and motor control, whereas the networks associated with gender were more widely distributed throughout the brain and tended to be involved in cognitive abilities such as attention, social cognition, and emotional processing.

The team’s findings do suggest neuroscientists need to consider sex and gender separately in biomedical research. 

It’s possible, they contend, that lumping study participants together by sex might cause researchers to overlook the influence of other variables, such as gender, on human health and behaviour. The discovery that sex and gender influence the brain in different ways, they believe “could change the way we do science.”

Access the research here.