Boys' and girls' brains are different—but not always in the ways you might think.

A common stereotype is that boys develop more slowly than girls, putting them at a disadvantage in school where pressure to perform is starting ever younger. Another notion is that puberty is a time when boys' and girls' brains grow more dissimilar, accounting for some of the perceived disparities between the sexes.

Now, some scientists are debunking such thinking. Although boys' and girls' brains show differences around age 10, during puberty key parts of their brains become more similar, according to recent government research. And, rather than growing more slowly, boys' brains instead are simply developing differently.

"There's a lot of work right now trying to go beyond sweeping generalizations, like boys aren't as good at reading," says Jay Giedd, chief of Brain Imaging at the National Institute of Mental Health's Child Psychiatry Branch.

The NIMH, as part of a 20-year-old brain-mapping project, has been doing MRI scans of young people's brains, age 9 to 22. By measuring the thickness of the brain's cortex and how it changes over time, scientists have found that boys' and girls' brains, on average, differ significantly at age 9. But by the time the participants reached age 22, the brains of the two sexes grew more alike in many areas critical for learning. In general, most parts of people's brains are fully developed by the age of 25 to 30. The NIMH study, which involved 284 people, was published last year in the journal Proceedings of the National Academy of Sciences.