Although a leader in the field of ADD, Dr Brown accepts his views can be controversial. He has to contend with opposition. At the extreme end there are those who deny that there is any such thing. There are also sceptics who think that the condition is over diagnosed and over treated and that good parenting and behaviour modification will remove the symptoms. This, he says, is ignorance of the complexity and debilitating effects of a hidden danger.

'The trouble is that folks with ADD do some things very well and teachers and parents can mistake this as a problem of will power. It is not. Bright girls can sit at the back of the class and be called daydreamers, for example. Boys can be brilliant at sports but be unable to focus well enough to do even basic assignments. ADD does not discriminate against age, intelligence or socioeconomic status.' He tells the story of a boy who was the best ice hockey goalkeeper observers had ever seen, who was able to follow the puck around the rink every second of the game but was having enormous difficulty in school.

Stories are central to Dr Brown's work. Trained as a clinical psychoanalyst he first became sensitised to students who were struggling with ADD when he conducted a series of interviews with two intelligent high school students who told him of their own and their parents' frustration caused by chronic underachievement at school. 'Their descriptions of their experiences and not being able to focus, difficulties with organisation and regulating sleep, helped me realise that here were cases of 'can't' rather than 'won't'.'

It was then that he got lucky. 'I tried some medicine I had read about in the [ADD] literature and there was a great improvement after only 48 hours. It got me thinking about just what is the chemistry of motivation? What is going on inside the brain of someone with ADD?'

He also had a personal interest. His own son David was, in his words, high maintenance: a boy who was curious, loved to talk but could be fretful, anxious and impatient. 'He needed lots of time: he had a voracious appetite for learning but we worried about him.'

Elements of hyperactivity and attention deficit have been known since the beginning of the 20th century. 'But 1980 saw a paradigm shift,' says Dr Brown. 'Diagnosis moved away from notions of brain damage and simple hyperactivity to accept ADD with or without the hyper [active] element.' His own consultations with patients and research as well as careful analysis of the growing literature on how the human brain works led him to the belief that chemistry played a significant part.

'ADD is a development impairment of the executive functions of the brain. These can be caused by accident - say a blow to the head - but in most cases it is a chemical deficiency. Imagine a symphony orchestra in which individual players know their part, but there is no conductor. Patients with this deficiency find it impossible to focus: they cannot voluntarily manage the complex dynamic neural process of selecting and engaging what it is important to notice, to do and remember moment to moment. One of my patients called it 'impotence of the mind'.'

After further research he produced a modified version of diagnostic rating scales for people over 12 years old and five years later for those identifiable from the age of three. 'Symptoms may be masked in children in particular as a range of adults provide scaffolding to help children develop. But teachers and parents should be aware that 6 to 8 per cent of all children can be diagnosed - and that could be a couple of children per class - and 4 to 5 per cent of adults. Boys are generally over diagnosed because of disruptive behavioural problems.'

So what are the signs to look for? Dr Brown identifies six major categories. 'First there is the inability to organise, prioritise and get started on work. Then there are the difficulties in focusing, shifting and sustaining attention. Parents should also look out for children who have problems with regaling alertness, effort and processing speed. Do children manage frustration effectively or modulate their emotions appropriately? Do they have difficulty holding one or several things in mind when busy? And finally, can children behave appropriately? We have to identify people early: inattentive first graders are three times more likely to have significant reading difficulties.'

But ADD is not an all-or-nothing condition. These six elements may be present to a greater or lesser extent. It is the skill of the clinician and the ability to use the latest delivery systems for methylphenidate - a drug used for decades - that are crucial. The drug helps essential chemicals to jump gaps in the brain's synapses with few or no known recorded clinical side effects.

'The latest time release systems have revolutionised treatment and can last up to 10 hours meaning that support is not withdrawn when the patient is at home, trying to do homework for example. Medicine works for over three-quarters of patients.'

Having ADD is not an insurmountable barrier to success and it is something that families can look out for at whatever age. Family genetic transmission is common. Dr Brown tells the tale of the son of a chief executive of a large international company diagnosed with ADD having just failed his first year of university. Dad returned after six months for a consultation of his own. He too had flunked university and admitted that without his three assistants he would never have been so successful. He wanted treatment so that he could return to complete his course on retirement. It's never too late.

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His latest book: Attention Deficit Disorder - The Unfocused mind in Children and Adults is published by Yale University Press