Nerve fiber tracts in the brains of autistic girls are more fragmented than those of typical girls. By contrast, autistic boys’ brain structure is indistinguishable from that of typical boys, a new study suggests1.
Compared with controls, autistic girls and women have less integrity in several nerve tracts, the researchers found, including in one that connects the occipital lobe at the back of the head to the temporal lobe at the side. The differences are greater on the left side of the brain than on the right. This may explain some of the girls’ challenges with language, because the brain’s main language areas are in the left hemisphere, Jou says.
The autistic girls have autism traits similar to those of the boys: The two sexes scored similarly on a test of autism severity. That result fits with a theory called the female protective effect, the researchers say. This theory posits that biological factors — such as a lack of integrity of brain tracts — must be more extreme in girls than in boys to result in autism.
The evidence is indirect, however.
“It’s consistent with the female protective effect, but I don’t think it’s evidence for [the theory],” Nordahl says.
The wide age range of the participants is a limitation of the study, Nordahl says. Structural differences in autistic boys and men could show up only at certain ages, and lumping people of disparate ages could mask this variation.