Now, along with neglectful parents, drug addiction and unemployment, magistrates' courts up and down the country could soon be ringing with the defence: "It wasn't me, it was my hormones."

Scientists at Cambridge University have made a link between reduced levels of the stress hormone known as cortisol in adolescent males and a tendency towards unruly behaviour.

The report is unlikely to find much favour with those who favour a good clip behind the ear as a solution to misbehaviour, but according to those behind the findings in some cases violent impulses can be linked to a chemical imbalance in the brain.

"Usually when people find themselves sitting in an exam room or going into hospital they become anxious and their levels of cortisol rise," says Dr Graeme Fairchild, who led the research with Professor Ian Goodyear. "The knock-on effect is that it makes people behave more cautiously. The hormone regulates their emotions, particularly in terms of temper or violent impulses.

"However, from the work we have done it seems the same chemical reaction does not occur in some disruptive males.

"We are not saying it's true in all cases, but for some people anti-social behaviour may be a form of mental illness caused by a chemical imbalance in the brain and body."

During the study, funded by the Welcome Trust, the researchers recruited 165 14- to 18-year-old males from schools, pupil referral units and the Youth Offending Service and put them in a series of stressful situations. Their cortisol levels were monitored before, during and after the experiment and they were also asked to rate how anxious they felt.

"The teenagers who took part in the study were quite severely disruptive and many had already been to prison," adds Dr Fairchild. "The idea was to see how they reacted in a competitive situation and what effect their emotions had on their hormones.

"We asked them to play a video game against someone else and told them they could win money if they came first. The video game looked simple, but in fact it had been rigged to be impossible. Occasionally, the mouse would stick and the person they were competing against was also able to send them messages telling them how badly they were doing.

"It was very frustrating for them and afterwards we asked them how they felt. Many admitted they had been made to feel a bit of a fool and when the situation spiralled out of their control they became angry and upset.

"Those reactions are totally normally, but usually it coincides with a rise in cortisol. For some of these teenagers that didn't happen and that's where the problems potentially lie. Most people learn from situations and their experience tempers their behaviour, but for some this just doesn't happen and it can mean they lash out without thinking."

According to a Home Office study, almost threequarters of anti-social behaviour orders are given to offenders under the age of 21 and the bill for responding to reports costs the Government about £3.4bn a year. Add in the effect on communities where anti-social behaviour has turned estates into no-go areas and the ripple effect is felt far and wide.

Dr Fairchild himself admits testing the cortisol levels of every young male to spot potential troublemakers isn't practical, but the research may lead to a change in the type of treatments which
are offered.

"If we can figure out precisely what underlies the inability to show a normal response to stressful situations we may be able to design new treatments for severe behaviour problems,"
he says.

"Also those who do have normal cortisol levels also seem to respond better to counselling. It's not about denying therapy to those who don't, but in the future it could help us devise more effective treatments targeted at individuals.

"Surely any possible treatment for this disorder, which offers the chance to improve the lives of both the adolescents and the communities in which they live, should be investigated."