ADHD News & Research

Greater Size Variation Found in Male Brains Than in Female Brains

Variations in brain volume may help explain why disorders such as ADHD and autism are more often found in boys.

June 29, 2017

Boys are more likely to have brains that are larger or smaller than average, a new study says, while girls have less size variation. This could help explain why disorders like ADHD and autism are more prevalent in boys, researchers concluded.

The study, published June 20 in the journal Cerebral Cortex, examined MRI scans from 1234 children and young adults between the ages of 3 and 21. Comparing brain volumes revealed that male brains were more likely to be exceptionally large or exceptionally small; female brains were more likely to be of “average” size, within the population. These variations were already apparent at age 3 and remained mostly stable over time — indicating that they most likely were genetic in origin, researchers said.

The reasons for the greater variation in male brains wasn’t immediately clear, but researchers hypothesized that it may be related to the single X chromosome found in men.

“If this X chromosome contains a gene that is related to a smaller brain structure, you will therefore see this in all brain structures in men,” said lead author Lara Wierenga. “Women’s two X chromosomes mean a tendency towards the average. As a different X chromosome is active in one brain cell than the other, the extremes balance each other out.” It’s been hypothesized that typical “male disorders” like ADHD and autism — each more prevalent in boys than in girls — may be related to the single X chromosome.

Though the current study doesn’t look at school performance specifically, the results correlate with prior studies on academic performance by gender, Wierenga said. Boys tend to have wider variation in grades and academic achievement, she said, and, “I wanted to research whether we also see these differences in the brain, and how they are given shape in the brain.”

But while brain volume might factor in, she added, it’s likely not the only cause for boys’ varying school performance. “We suspect that environmental factors increase the differences between boys and girls in variation in school performance.”