Sex and gender are often conflated or equated in everyday conversations, and most American adults believe a person’s gender is determined by sex assigned at birth. But a new study of nearly 5,000 nine and 10-year-olds found that sex and gender map onto largely distinct parts of the brain.

The research gives a first insight into how sex and gender may have “measurable and unique influences” on the brain, study authors said, just as other experiences have been shown to shape the brain.

“Moving forward, we really need to consider both sexes and genders separately if we better want to understand the brain,” said Dr. Elvisha Dhamala, an assistant professor of psychiatry at the Feinstein Institutes for Medical Research and the Zucker Hillside Hospital in Glen Oaks, California, and a co-author of the study, published Friday in the journal Science Advances.

The researchers on the new study defined sex as what was assigned to the child at birth. In the US, clinicians make this assignment based on genitalia. Most people are assigned either female or male, according to the research; the rest are intersex, a person whose sexual or reproductive anatomy doesn’t fit this male/female binary.

The researchers defined gender as an individual’s attitude, feelings and behaviours, as well as socially constructed roles. They noted specifically that gender is not binary, meaning not all people identify as either female or male.

Both sex and gender are a core part of human experience. They’re key to how people perceive others and how they understand themselves. Both can influence behaviour as well as health, the study authors say.

The researchers looked at brain imaging data from 4,757 children in the United States, 2,315 assigned female at birth and 2,442 assigned male at birth, who were ages nine and 10 and were a subset of the Adolescent Brain Cognitive Development (ABCD) study, the largest long-term study of brain development and child health in the United States. Over a period of 10 years, the children in the ABCD study underwent comprehensive neuroimaging, behavioural, developmental and psychiatric assessments.

Beyond tests such as MRIs, the scientists did surveys of the children and their parents that were focused on gender, both at the beginning of the research and then a year later. The children were asked about how they expressed their gender and how they felt about it. The parents were asked about a child’s sex-typed behaviour during play and whether the child had any gender dysphoria, a term that mental health professionals use to describe clinically significant distress felt because a person’s sense of their gender does not match their sex the assigned at birth.

Parents were a key part of the study, said study co-author Dr. Dani S. Bassett, a professor at the University of Pennsylvania with appointments in the Departments of Bioengineering, Electrical & Systems Engineering, Physics & Astronomy, Neurology, and Psychiatry.

“When kids have a particular kind of gender behaviour or gender expression, that will influence how their parents and also other caregivers and friends and family … et cetera interact with them,” Bassett said. Information about a parent’s perception of a child’s gender gives researchers a better sense of the child’s social environment and how it may affect their brain development.

The authors used a kind of artificial intelligence called machine learning that built a model that could predict a child’s sex and reported gender from their brain scan. When the researchers looked the children’s brain scans, the results seemed to show that sex influenced different regions of the brain that are involved in visual processing, sensory processing and motor control and some regions involved in executive function, which lets an individual organize and integrate information across time.

Gender seems to influence some of the more sensory-specific networks that are associated with sex, but it also seems to have a broader influence and can be detected on different brain networks involved in executive function, including things like attention, social cognition and emotional processing.

“The fact that we’re able to capture how gender maps onto the brain basically just tells us that gender is influencing our brain,” Dhamala said.

The structure of the human brain can be shaped by expertise and experiences. Research on London taxi drivers — who must take extensive tests to show that they can navigate the city’s streets without maps or GPS — seems to show that they have significantly larger posterior hippocampi, the part of the brain related to spatial memory and navigation, than in people who aren’t taxi drivers.

“Similarly, as individuals and as humans, we are experts about ourselves and our genders. So it makes sense that gender will also be mapped within our brains,” Dhamala said.

What the new study cannot do is predict what gender a person may identify with beyond one limited snapshot in time captured by the scans and surveys. Gender, the authors note, is not something that is necessarily static, and a person’s understanding of their gender can change throughout their lifetime.

The study also can’t determine what things in someone’s environment will influence their brain function in terms of sex or gender, nor can it identify what a person’s sexual orientation might be.

“Sexual orientation is independent from gender and from sex,” Bassett said, and it may be differently mapped in the brain.

The researchers say they hope to someday learn more about how sex and gender interact in a person’s life and how they influence one another and the brain throughout a lifetime. They also hope to see how different cultures affect a person’s gender and their brain development.

A 2022 poll showed that most American adults — and the vast majority of conservatives — believe that a person’s gender is determined by the sex assigned at birth. The distinction is key to gender-affirming care, medical treatment for people who identify as a gender that is different from the one they were assigned at birth. Conservative politicians have pushed for a record number of bans on such care, and nearly half of U.S. states have enacted bans on gender-affirming care for minors.

The study did not look at whether sex or gender were congruent or incongruent in any study participant. Rather, it looked at the child’s binary sex and gender across self- and parent-reported measures. The study couldn’t provide any specific findings if sex and gender were incongruent.

“Going forward, the hope is that we can motivate other scientists to consider science and gender in their analyses in the data collections in their programs and research,” said study co-author Dr. Avram Holmes, an associate professor of psychiatry at Rutgers University.

The field of neuroscience has only just begun to acknowledge and address the presence of biases and barriers to inclusivity within research, Holmes said.

A fuller understanding of the way the brain works in terms of sex and gender could also have practical implications and potentially help scientists find better ways to treat people with brain-related illnesses. For instance, the study pointed to how people assigned male at birth are more likely to be diagnosed with substance use and attention deficit disorders.

“It’s not that sex and gender necessarily drive illness rates, but the cultures people are embedded in can also influence the likelihood they may or may not develop a particular illness,” Holmes said. “So the types of environmental pressures a child undergoes across development could increase or decrease their risk for experiencing illness, independent of their initial brain biology.”