Two decades ago, a landmark study showed that the brains of kids with attention-deficit/hyperactivity disorder (ADHD) take longer to mature. But new research suggests that this result, which was based on brain scans from a few hundred children, was a mirage.

What was thought to be a hallmark of the ADHD brain, the study found, instead reflects average sex differences in how the brains of boys and girls develop over childhood. The earlier dataset, which used a smaller sample size, may have become skewed to more closely reflect the average boy's brain development, the new research suggested.