From the time you’re born until about age 2, the outer layer of your brain — the cortex — rapidly thickens in a frenzy of neuron formation. After all that excitement, that dense hedge of nerve cells was trimmed back in a process called “cortical thinning.”
Now, a new study has found some key differences in how this process occurs in autistic children, depending on where they were born.
Previous studies have found variations in the way the brains of autistic children undergo cortical thinning, but so far the picture is hazy and inconsistent. This is partly because, historically, studies of autism spectrum disorders have underrepresented the female gender, and this is true for research on cortical development.
“It’s clear that this sex bias is partly due to the underdiagnosis of autism in women,” says neuroscientist Christine Wu Nordahl of the University of California Davis. “But this study suggests that differences in diagnosis aren’t the whole story—there are also biological differences.”
Although the true ratio is likely to be much lower, only one woman is diagnosed with autism for about every four men who receive a diagnosis, suggesting the possibility that gender influences the development of the condition.
By including both autistic and non-autistic children in the study, the researchers could compare differences in cortical thickness associated with autism within each gender group at birth (for example, the difference between autistic and non-autistic females) and also compare the results. for autistic groups only based on sex at birth.
The study involved brain scans of 290 autistic children (202 males, 88 females) and 139 non-autistic children (79 males, 60 females) with typical development.
Collected up to four times for each child between the ages of 2 and 13, these scans provided a detailed picture of the development of a child’s cortex from the age when the cortex is thickest to the age when thinning is most rapid. , usually around 14 years.
At age 3, certain areas of the cortex—about 9 percent of its total surface area—were thicker in autistic females than in undiagnosed peers of the same age and sex. In the 3-year-old male group, there were few significant differences in cortical thickness between autistic and non-autistic children.
By age 11, cortical gender differences were much harder to discern. The main differences revealed in the study were only visible as changes in the cortex over time.
Compared to their non-autistic counterparts, children with autism had faster cortical thinning in certain areas during childhood, while autistic males had less rapid thinning overall than non-autistic males. These changes were not consistent throughout the brain: only in certain cortical areas that make up less than 5 percent of its totality, including networks that plan and control motor tasks, maintain attention and solve problems, and the brain’s “radar” that helps us focus , when our terms change.
In other areas, such as the limbic network, where behavioral and emotional responses arise, cortical thinning occurred more rapidly in autistic males compared to non-autistic males and less rapidly in autistic females compared to non-autistic females.
There are many reasons why a person’s biology may relate to or reflect what sex they were assigned at birth, and it is not necessarily fixed: some traits may be linked to the X or Y chromosome, while others are influenced by different hormone levels or even be the result of cultural attitudes towards assigned sex and gender that lead to different behaviors and lifestyles.
So while this study found observable differences between the male and female groups, more detailed research will be needed to understand exactly how these differences arise and what this might mean for transgender, nonbinary, or intersex people with autism.
This nuance is particularly relevant here, given that gender-queer adults are up to six times more likely to be diagnosed as autistic than cisgender adults (those who identify with their gender and the gender assigned at birth).
“We usually think of gender differences as larger after puberty. However, brain development between the ages of 2 and 4 is highly dynamic, so small changes in the timing of development between the sexes can lead to large differences that later converge,” says the psychiatric researcher Derek Andrews of the University of California Davis.
“It is important to learn more about how gender differences in brain development may interact with autistic development to lead to different developmental outcomes for boys and girls.”
This research was published in Molecular psychiatry.