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Sex and gender are often confused or equated in everyday conversations, and most American adults believe that a person’s gender is determined by the sex assigned at birth. But a new study of nearly 5,000 9- and 10-year-olds found that sex and gender reside in largely separate areas of the brain.
This research provides a first look at how sex and gender can have “measurable and unique influences” on the brain, the study authors said, just as other experiences have been shown to shape the brain.
“Going forward, we really need to consider the two sexes and the two genders separately if we want to better understand the brain,” said Dr. Elvisha Dhamala, assistant professor of psychiatry at the Feinstein Institutes for Medical Research and Zucker Hillside Hospital in Glen Oaks, California, and co-author of the study, published Friday in the journal Science Advances.
The researchers in the new study defined sex as the sex assigned to a child at birth. In the United States, clinicians make this assignment based on genitalia. Most people are either female or male, according to the study; the rest are intersex people, whose sexual or reproductive anatomy doesn’t fit that binary male/female distinction.
Researchers have defined gender as an individual’s attitudes, feelings, and behaviors, as well as socially constructed roles. They have noted that gender is not binary, meaning that not all people identify as female or male.
Sex and gender are both essential parts of the human experience. They are central to how people perceive others and understand themselves. Both can influence behavior as well as health, the study authors say.
The researchers examined brain imaging data from 4,757 children in the United States, 2,315 female at birth and 2,442 male at birth, who were 9 and 10 years old and part of a subset of the Adolescent Brain Cognitive Development (ABCD) study, the largest long-term study of children’s brain development and health in the United States. Over a 10-year period, children in the ABCD study underwent comprehensive neuroimaging, behavioral, developmental and psychiatric evaluations.
In addition to tests like MRIs, the scientists conducted gender-specific surveys of the children and their parents at the beginning of the study and a year later. The children were asked about how they expressed their gender and how they felt about it. The parents were asked about their child’s gendered behavior during play and whether they had gender dysphoria, a term mental health professionals use to describe clinically significant distress felt because a person’s perception of their gender doesn’t match the sex they were assigned at birth.
Parents played a key role in the study, said study co-author Dr. Dani S. Bassett, a professor at the University of Pennsylvania, lecturer in the departments of bioengineering, electrical and systems engineering, physics and astronomy, neurology and psychiatry.
“When children have a particular type of gender behavior or expression, it influences how their parents, as well as other caregivers, friends and family, etc., interact with them,” Bassett said. Information about parents’ perceptions of their child’s gender gives researchers a better idea of a child’s social environment and how it may affect their brain development.
The authors used a type of artificial intelligence called machine learning that built a model that could predict a child’s sex and tell them their gender based on their brain scan. When the researchers looked at the children’s brain scans, the results appeared to show that gender influenced different brain regions involved in visual processing, sensory processing, and motor control, as well as some regions involved in executive function, which allows an individual to organize and integrate information over time.
Gender appears to influence some of the more sensory-specific networks that are associated with sex, but it also appears to have a broader influence and can be detected across different brain networks involved in executive function, including things like attention, social cognition, and emotional processing.
“The fact that we are able to capture how gender is integrated into the brain simply tells us that gender influences our brain,” Dhamala said.
The structure of the human brain can be shaped by experience and expertise. Research on London taxi drivers, who must pass extensive tests to prove they can navigate the city’s streets without a map or GPS, appears to show that their posterior hippocampi, the part of the brain linked to spatial memory and navigation, are significantly larger than those of people who aren’t taxi drivers.
“Similarly, as individuals and as human beings, we are experts on ourselves and our gender. So it makes sense that gender is also mapped in our brains,” Dhamala said.
The new study doesn’t predict what gender a person might identify with beyond a limited snapshot in time captured by scans and surveys. Gender, the authors point out, is not necessarily static, and a person’s perception of their gender can change throughout their life.
The study also cannot determine what elements of a person’s environment will influence their brain functioning in terms of sex or gender, nor identify what a person’s sexual orientation might be.
“Sexual orientation is independent of gender and sex,” Bassett said, and it may be mapped differently in the brain.
Researchers hope to one day learn more about how sex and gender interact in a person’s life and how they influence each other and the brain throughout life. They also hope to see how different cultures affect a person’s gender and brain development.
A 2022 poll found that most American adults—and the vast majority of conservatives—believe that a person’s gender is determined by the sex they were assigned at birth. That distinction is critical to gender-affirming care, which is medical treatment for people who identify with a gender different from the one they were assigned at birth. Conservative politicians have pushed for record bans on such care, and nearly half of U.S. states have enacted bans on gender-affirming care for minors.
The study did not seek to determine whether or not participants were sex or gender congruent. Instead, it examined the child’s binary sex and gender through self-reported and parent measures. The study could not provide specific results if sex and gender were incongruent.
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“In the future, we hope to motivate other scientists to consider science and gender in their analyses and in the data collection of their programs and research,” said study co-author Dr. Avram Holmes, associate professor of psychiatry at Rutgers University.
The field of neuroscience is only just beginning to recognize and address the presence of biases and barriers to inclusion in research, Holmes said.
A better understanding of how the brain works based on sex and gender could also have practical implications and help scientists find better ways to treat people with brain diseases. For example, the study found that people assigned male at birth are more likely to be diagnosed with substance abuse and attention deficit disorders.
“Sex and gender don’t necessarily drive disease rates, but the cultures in which individuals grow up can also influence whether or not they’ll develop a particular disease,” Holmes said. “So the kinds of environmental pressures a child experiences during development can increase or decrease their risk of developing disease, regardless of their initial brain biology.”