Genetics key to teen violence

Whether a criminal teenager turns into a violent adult or grows out of crime, may be related to how low his ears are set or the types of food he was given as a child.

International research shows antisocial behaviour in young adults can be written into their genetic code, and made worse by bad parenting.

Indicators that an antisocial child may turn into a life-long violent criminal can be picked up in kindergarten, according to research summarised in this week's New Scientist magazine.

Of the 535 males and 502 females born in New Zealand in 1972 and 1973 who were signed up at birth to the University of Dunedin's Multidisciplinary Health and Development Study, those who dabbled in crime as a teenager can be divided into two clear groups, Terrie Moffitt from the Institute of Psychiatry in London said.

But the more problematic type had biological predispositions to behaviour problems, the signs of which could be picked up as young as three years of age.

These children - more often boys - tended to have a low IQ, poor language skills, and were often diagnosed with attention-deficit hyperactivity disorder.

Minor physical anomalies such as low-seated ears or furrowed tongues - possibly a sign of poor neural development or damage - could also be signs.

Combined with bad parenting, poverty or abuse, these children were at greater risk of turning to a life-long criminal career, she said.

The early onset group accounted for only 10 per cent of the Dunedin males, but by the age of 26 they had accrued almost half of the violent convictions for the entire study.

Ms Moffitt's study of the Dunedin children also focused on the activity of an enzyme linked to aggression in both animals and humans.

By itself, the activity of the enzyme had little influence, but if the boys were more predisposed to aggression and had suffered some abuse as children they were three times as likely to be diagnosed with conduct disorder in adolescence and 10 times as likely to have been convicted of a violent crime in adulthood.

A US study has found there is a worrying subset of kids within the early onset group who do not react emotionally at all, scoring high in tests used for diagnosing adult psychopathy.

A study at the University of New Orleans found the 30 per cent of early onset antisocial children who showed these traits were most likely to turn to life-long violence.

Children in this group lack empathy and guilt, are thrill-seeking, fearless and narcissistic, says psychopathy expert James Blair from the National Institute of Mental Health in the United States.

Learning to fear punishment or recognise someone else's fear or sadness is difficult for psychopaths, he said.

"If they want something and punching someone in the face is the way to extract it, they might be more likely to engage in that kind of behaviour," he said.

Mr Blair says there are few signs that these psychopathic traits are caused by external factors like poor parenting or abuse, but they could be triggered by social forces like poverty.

The generally accepted approach to tackling the problem of antisocial and violent children is to intervene as young as possible with improved parenting.

Some experts say better parenting could even begin before birth.

A trial of monthly nurse visits throughout pregnancy and until the child's second birthday, in New York in the late 1970s, has been praised by scientists at the University of Colorado.

By the age of 15, the 315 children involved in the trial had only half of the number of arrests, one fifth the number of convictions, smoked and drank less and were less promiscuous than their untreated contemporaries.