Attention-deficit/hyperactivity disorder (ADHD)

Attention-deficit/hyperactivity disorder, which most people know as ADHD, is a developmental disorder. That means it shows up while a person is still developing: it starts in childhood. It's recognized by inattention, which is trouble paying attention; hyperactivity, which means having too much energy or moving and talking too much; and impulsivity, which is acting without thinking or having a hard time with self-control. In ADHD, these don't come and go. They're persistent, there day in and day out, and they can get in the way of everyday life.

The cause is still uncertain. But one imaging study, a kind of study that takes pictures of the inside of the brain, did show how the brain matures in young people with ADHD: it follows the normal pattern, but some regions lag behind, especially the frontal cortex. The cortex is the brain's outer layer, and the part at the front is important for controlling attention and planning. Picture houses built from the same blueprint and going up in the same order, where certain rooms, mostly the ones in front, are running behind. The house isn't following a different blueprint; that part of the job is just behind schedule.

And here's the key point: that delay showed up only when a very large number of children were included in the study. It couldn't be seen in the brain scans of a single person. It's like noticing that the students at one school are, on average, taller than the students at another: the difference jumps out when you measure lots of kids, but not when you look at just one child.