A couple of hours spent with three-year-old boys is enough to pick out the ones most likely to end in jail for violent offences, says research from New Zealand.
It shows that a "life course" is mapped out for 5 per cent of boys: from pre-school aggression to anti-social acts such as vandalism, to violent offending by the age of 21.
The findings were revealed at a London conference on the roots of violence in children and young people at the Royal Society of Medicine.
Professor Terrie Moffitt of London's Institute of Psychiatry detailed results of a longitudinal study carried out in the New Zealand town of Dunedin. Her findings confirm the vital importance of intervention before the age of three.
Every second year since 1973, researchers have interviewed all the children and young people born in that year in the town. More than 95 per cent of the original sample are still taking part in the survey.
All boys have a high risk of getting into trouble in their teens, the study claims, with a 20-25 per cent rate of "fairly minor" anti-social acts recorded. But if aggressive three-year-old boys go on to commit minor misdeeds in their early teens, they are almost inevitably destined for a "persistent life course" of violence. Boys from difficult families, typically including violence, poverty and alcoholism, who showed low IQ and a "lack of control" in an interview lasting a couple of hours at the age of three, have twice the rate of conviction for serious, violent offences at age 21 as their peers.
The picture is even gloomier for a minority of girls, said Professor Moffitt. Only 4 per cent showed awkward behaviour at age three and they rarely went on to commit any violent crimes.
But half of those anti-social toddlers were in violent relationships by the age of 21, a third had had babies by the age of 19, and all the teenage mothers were beaten by their partners. The cycle had already begun again.