A simple saliva test may predict which boys will have some of the worst behavioural problems, according to experts.

This means it may be able to pick out those who are most at risk to progress to a life of violent crime.

Boys aged seven to 12 with low levels of the stress hormone cortisol were linked with "extremely aggressive behaviour".

They were also frequently voted the "meanest" child by their classmates.

Psychiatrists from the University of Chicago theorise that the lack of the stress hormone makes the boys far less likely to fear retribution for their aggressive behaviour.

Professor Keith McBurnett, who led the study, said: "It suggests there is a lasting biological and not just parental or short-term environmental component to this type of chronic antisocial behaviour.

"Children with persistent conduct disorder may have genes which predispose them to produce certain hormones differently, or their hormone production may have been altered before or soon after birth."

Children with what is defined as "persistent conduct disorder" tend to remain disruptive for decades - and may go on to carry out violent crime.

Schoolyard fighting

The researchers followed 38 boys who had been referred to the psychiatric clinic for disruptive behaviour.

All had similar IQs, socioeconomic and ethnic characteristics.

They collected saliva samples at two-yearly intervals, and assessed their tendency towards aggression, with symptoms such as starting fights, using weapons, being cruel to people or animals, stealing, or even forcing sexual acts.

And all the children were evaluated by their peers - the fellow students were asked to name the three boys in class who fought most.

The 12 children who persistently had the lowest cortisol levels averaged 5.2 symptoms of conduct disorder - the rest averaged only 1.5 symptoms.

And 11 of the 12 boys with the lowest readings developed aggressive symptoms before the age of 10.

The precise mechanism for the action of cortisol is unknown - it is normally secreted when someone faces a threatening or stressful situation.