In just fifty years attention deficit hyperactivity disorder (ADHD) has emerged from obscurity to become by far the most common neuropsychological disorder of childhood. It did not even merit mention in the standard textbooks of childhood psychiatry of the 1950s, but now virtually everyone is familiar with its salient features: ‘He gave every impression of being a perpetual motion machine,’ writes Matthew Smith of a boy in his first class as a student teacher. ‘He bounced around the classroom, sharpening his pencil, looking out of the window, trying on the other kids’ boots … It was as if the notion of sitting quietly at his desk was simply a complete anathema.’

This boisterousness is not just trying for all concerned, but predictably associated with several adverse consequences – academic under-achievement, defiance and aggression when thwarted, failure to ‘engage’ with others and, in adult life, an inability to settle into a regular job.

The simple but obvious explanation for the remorseless rise in