What is ADHD, and how do doctors describe it?

ADHD, short for attention deficit hyperactivity disorder, is a neurodevelopmental condition, which means it shapes how a brain develops from early life. Doctors describe it as a pattern of inattention, hyperactivity and impulsivity, sometimes with strong swings of emotion, that is stronger and more widespread than expected for a person's age and that gets in the way of daily life.

It is not defined by a single behaviour. Under the fifth edition of the American diagnostic manual, DSM-5, a clinician looks for a set of symptoms that have lasted at least six months and are much greater than in others of the same age. At least six are needed for someone under 17, and five for someone 17 or older. They must show up in at least two settings, such as home and school or work, and they must really interfere with how a person functions. Several of them must also have been present before age 12.

The look of ADHD changes with age and between people. Hyperactivity tends to fade into an inner restlessness in teenagers and adults. Girls and women often show fewer hyperactive and impulsive signs and more inattention, which can be mistaken for being spacey or forgetful. Many children with ADHD can pay close attention to things they find interesting, so the label is about difficulty directing attention, not an absence of it.

The idea is old. In 1798 Sir Alexander Crichton wrote of "mental restlessness" and children with the "fidgets", and a clear description is credited to George Still in 1902. How many people are said to have ADHD depends heavily on the rulebook used to count them.