“Does the brain have a sex?” This question has been the focus of much research for decades… and the subject of scientific controversy. The cause is a still very imperfect understanding of our brain, even though research on the interactions between sex, neurobiology and behavior has exploded over the last twenty years. Many studies report contradictory results or cannot be reproduced. It is nevertheless accepted that, if the brains of men and women are globally similar, there are some differences. The most notable being that the brain of men is on average larger than that of women, by around + 6%, even at equal size. A study published in July 2021 in Human Brain Mapping also shows that a few hundred brain regions have slight statistical differences in size.
But neuroscience – and biomedical research in general – remains helpless to explain the causes and consequences of these differences. Are they linked to genetic factors, hormonal or environmental differences, to education or to the cultures of societies? No one knows precisely. There are hardly any more answers concerning the consequences. All we know is that even if there is a correlation between brain volume and intelligence quotient, men are not more “intelligent” than women. The IQ scores of the two sexes are very close. To top it all off, we must also take into account brain plasticity, i.e. the fact that our personal experiences modify the structure of our brain. For example, it is known that in pianists, the areas of the cortex that control the fingers and hearing are thicker than in the average human being.
Different brain networks depending on sex, but also gender
As if that were not enough, researchers have decided to ask themselves if our brains have… A gender! American neuroscientists have just published a study in Science Advance, this Friday, July 12. “The growing literature on sex differences and the lack of reproducibility of many of the reported differences suggest potential biases and/or misunderstandings in how we study, interpret, and report sex-related findings,” the authors write in the introduction. They tried to add to this by analyzing brain imaging data from thousands of American children. According to the researchers, their results indicate that there are differences in brain networks based on the children’s sex, but also, sometimes, their gender. In their work, the researchers specify that the “sex” of the children in the study corresponds to the attribution of the biological trait that was determined at birth (on average, it results in the designation male or female in 99.95% of cases); and that “gender” corresponds to “the children’s self-reports and parental statements” and was not considered binary or a specific category.
To try to disentangle the effects of sex and gender on brain network activity (which indicates how active and connected different parts of the brain are), the researchers analyzed MRI data from 2,315 girls and 2,442 boys aged 9 to 10 in the cohort. Adolescent Brain Cognitive Development study, a longitudinal study underway in the United States that aims to understand the development of the child’s brain. “We used this data to see how different regions of the brain communicate and form networks,” explains Elvisha Dhamala, a neuroscientist at Northwell Health Feinstein Institutes for Medical Research in New York (United States) and lead author of the study. She and her team then analyzed the links between these networks and the sex assigned at birth, then with the gender declared by the children and parents. “Our results show that sex and gender each influence different brain networks,” says the researcher.
To determine this, she and her team mapped different regions of the brain that were linked, or not, to biological sex. They also used machine learning algorithms to determine whether the connectivity patterns of brain networks could identify the biological sex and gender of each participant. By analyzing certain brain networks, for example, the algorithms were able to predict the biological sex of children with very high accuracy. And by analyzing distinct brain networks, they sometimes also successfully predicted gender. The researchers then compared and analyzed their different results, notably using statistical tests, and determined that sex and brain networks are indeed linked to sex, but also to gender.
“In principle, this study does exactly what needs to be done: jointly studying sex and gender in order to determine what is the difference between one and the other,” says Franck Ramus, research director at the CNRS. in cognitive sciences at the Ecole Normale Supérieure in Paris, a specialist in child cognitive development, who did not participate in the study but was able to consult it for L’Express. This specialist in child cognitive development, emphasizes that, in this study, the only gender measurement predicted by brain activations is that provided by the parents (who answered whether the child’s behavior was typical of his or her sex) and not that provided by the children (who answered four questions about their gender identity). “So strictly speaking, the authors did not find a brain correlation of gender identity independent of sex. But this may of course be due to the young age of the children and the low number of trans people declared at this age,” he specifies.
Gender: cause or consequence of these differences?
This finding is all the more interesting because while some studies have begun to focus on the influence of sex on the human brain and behavior, “the contributions of gender are unknown,” the authors point out. Yet such knowledge could help future research on the neurobiological diversity of brain development, but also of its influence on behavior and cognition, and thus its implications for the sociocultural definition of gender itself. It could also guide further research on various brain disorders, many of which show sex differences in prevalence, onset, and clinical presentation. “Understanding the neurobiological underpinnings of sex and gender is crucial for the further identification of how sex and gender influence health and disease and for the development of sex-specific and gender-oriented diagnostic and prognostic tools,” confirms Elvisha Dhamala.
Given the potential applications of these results, but also the windows they open onto sometimes controversial debates, the researchers nevertheless want to exercise caution. They thus emphasize that their analyses are limited to demonstrating that sex and gender influence brain networks. “We did not study the cause-and-effect relationships between the functioning of brain networks and sex or gender, so we cannot say whether these networks are a cause or a consequence,” specifies the lead author of the study. She and her team also emphasize certain important limitations of their work, in particular the fact that it is based on “a snapshot” of a stage in the lives of these children who, moreover, all live in the United States and are therefore immersed in a particular culture, codes and traditions.
Finally, they call on the scientific community to replicate their results and to push forward research to better understand how brain network associations with sex and gender may evolve over the course of life and whether the effects may vary in different sociocultural environments. “But if our results are confirmed, it will be clear that neuroscience must take into account sex and gender differences in future brain studies, particularly when collecting, analyzing and interpreting data,” the authors explain.
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