Its victims suffer from poor concentration, an inability to sit still, and can cause disruption in even the most well-ordered class. One in 20 children in Britain is said to have it, yet there is no consensus over what causes it, how best to treat it, or even whether it exists at all.

Attention deficit hyperactivity disorder (ADHD) attracts controversy like few other medical conditions. Depending on your point of view, the condition is either the root cause of much of the bad behaviour in schools or a convenient label that allows parents to escape responsibility for their children’s behaviour.

A TES poll this week found that about 10 per cent of heads and classroom teachers are sceptical about the disorder.

But one thing that cannot be denied is the problems caused by and suffered by children diagnosed with ADHD, also known as “hyperkinetic disorder”.

The list of symptoms reads like a teacher’s nightmares: restless overactivity; frequent interruptions; poor social skills; mood swings; clumsiness; forgetfulness; and a track record of indiscipline.

But the cause of these problems remains largely a mystery.

Explanations range from a chemical imbalance in the brain to deficiencies in children’s diet. Brief pauses in breathing while sleeping (sleep apnoea) and mothers smoking during pregnancy have also been blamed.

One theory, known as “hunter-versus-farmer”, explains many symptoms as the result of natural selection. Put simply, Thom Hartmann, a United States radio host and author of eight books on the disorder, suggests that because we have moved from a predominantly hunting society towards farming and civilisation, so genes have evolved to suit their changing circumstances.

People with the condition, he argues, have inherited a collection of genes selected at a time when hunting was of paramount importance.

Lack of understanding about what causes the disorder has not prevented it becoming one of the most widely diagnosed psychiatric disorders among children.

Almost 400,000 children in the UK are believed to have ADHD. The number diagnosed with the condition is somewhat lower, but it is increasing rapidly.

The number of prescriptions for methylphenidate hydrochloride, the drug used to treat ADHD, rose from 271,000 in 2002-03 to 329,000 the following year, an increase of more than 20 per cent.

The worldwide market for these drugs is now estimated at more than pound;2 billion.

Despite this increase, some researchers - notably Professor Peter Tymms and Dr Christine Merrell, of Durham university - worry that too many children, particularly girls, who have ADHD are not being diagnosed, and so do not receive help.

Their research shows pupils with ADHD start school in the bottom quarter of the population for reading and mathematics but have fallen further behind - to the bottom ninth - by the time they leave primary school.

The study of 5,500 pupils found early identification is vital to ensure appropriate teaching techniques can be put in place in order to close this achievement gap.

For parents and teachers, the diagnosis of ADHD and the subsequent prescription of drugs can be a huge relief. But some experts worry that labelling children in this way helps parents and schools evade their responsibilities.

In his forthcoming book, David Cohen, professor of social work at Florida international university, says: “To millions of families, the label (ADHD) provides a legitimate justification to ‘outsource’ some responsibilities related to raising children... “Messages about ADHD destined for parents have strong guilt-dissolving, natural calamity components, as in: ‘Learning that your child has ADHD can be distressing. But ADHD is nobody’s fault. Nothing you or your child has done has caused it.’ The label also provides schools with yet another alibi to explain why they regularly fail to make some children fit in.”

Such concerns are exacerbated by concerns over the long-term effects of the drug methylphenidate hydrochloride, commonly known by the brand name Ritalin.

Bizarre as it seems, the drug given to hyperactive children is a derivative of amphetamines, or so-called “speed”.

According to the New Scientist, methylphenidate hydrochloride and speed have different chemical structures but similar effects on behaviour. The increased concentration and wakefulness it induces has led to a thriving black market for the drug in the US, where students take it to boost their performance in exams or to party late with friends.

The downside includes problems such as difficulties with eating and sleeping, headaches and tics. Perhaps most worrying is the risk of “behavioural rebound”.

Gwynedd Lloyd, head of educational studies at Moray House school of education, in Edinburgh, who is co-editing Critical New Perspectives on ADHD with Professor Cohen, says that when children come off Ritalin their problems can reappear in a more severe form. This increases the need for the medication, creating a vicious circle of drug use and bad behaviour.

Many parents beg to differ.

Andrea Bilbow, whose 18-year-old son was diagnosed with ADHD while at primary school, believes disputes over medication and the condition itself are depriving children of the support they desperately need.

Ms Bilbow, who is the founder and chief executive of the National Attention Deficit Disorder Information and Support Service, said: “Many teachers are struggling without the support they need from heads, SENCOs (special educational needs co-ordinators) and local authorities.

“For children who take it (Ritalin) it ensures they get their education and keep their friends.”

Official guidelines stress that using medication to relieve the symptoms should be a last resort and is no substitute for a cure.

Nevertheless, the number of prescriptions continues to snowball. An inquiry in Scotland was announced in December after a tenfold increase in the use of methylphenidate hydrochloride.

Is it time for a similar inquiry south of the border?

Certainly, concerns that Ritalin is a blunt tool being used to solve an ill-defined problem refuse to go away.

Francis Fukuyama, professor of political economy at Johns Hopkins university in Baltimore, Maryland, and a member of the US president’s council on bioethics, has questioned whether ADHD as a discrete condition exists at all.

He said: “ADHD is actually not a disease at all, but merely the tail of a normal distribution of behaviours related to overactivity and lack of concentration.

“Character, it was once believed, was something that had to be shaped through self-discipline, struggle and a willingness to confront discomfort and wrong inclination; now, we have a medical short-cut to the same result.”

Sceptics say we should look to reduce the effects on children of changing family life, parents’ longer working hours and the effect of television and computer games on the attention-span of young children.

The truth is we know very little with certainty about ADHD. But it is certainly easier to give amphetamine-like drugs to young children rather than re-examine the way society treats its children.

But are we really sure this is a good idea? Other countries such as France and Italy appear to have resisted using medication.

For Gwynedd Lloyd, simply dishing out drugs is not the answer. She said:

“In 10 years’ time, people will look back and ask, ‘What were we thinking?’”

Platform, see opposite

ADHD: some key characteristics and strategies

Does a child in your class have ADHD? Symptoms include:

* A record of indiscipline

* Aggression, loss of control or mood swings

* Restless overactivity

* Impulsiveness and impatience

* Frequent interrupting

* Distraction and inattention

* Forgetfulness

* Difficulty with reading and other key skills

* Poor social skills and low self-esteem

* Clumsiness and poor co-ordination

* Poor memory skills

Dol Seat the child away from possible sources of distraction

* Use eye contact and short sentences

* Be willing to restate rules. Try to praise positive behaviour and avoid public criticism

Don’t l Make the child look needy or the subject of favouritism

* Keep children in over breaktime - it is unlikely to help them or you

* Get into confrontational situations - children with ADHD respond best to calmness

Further advice for teachers about how to help children with ADHD is available at www.teachernet.gov.ukteachingandlearninglibraryadhd