Testing for intelligence, motor skills and troubled behavior at age three showed which tots were likely to wind up behind bars or as deadbeat dads later in life, a long-term study reported Monday.

The results should make it possible to deliver help to young kids at risk before they wind up with a rap sheet or a drug addiction, said the study, which measured the social cost of early childhood privation.

The findings published in the journal Nature Human Behavior may also apply to other wealthy societies with comparable income inequalities and social safety nets.

"Most expenses from social problems are concentrated in a small segment of the population," said Avshalom Caspi, lead author of the study and a professor of psychiatry and neuroscience at Duke University in the United States.

"We can predict this quite well, beginning at age three, by assessing a child's history of disadvantage, and particularly 'brain health'," he noted in a statement.

In a long-term experiment involving 1,000 people, the first of a dozen evaluations from age three to 38 measured IQ, language and motor skills, and rated the children for their tolerance of frustration, as well as restlessness and impulsive behavior.

The study revealed other concrete measures of the cost to society generated by a minority born into difficult circumstances.

The troubled 20 percent accounted for three quarters of fatherless child-rearing and drug prescriptions, as well as more than half of hospital nights and cigarettes smoked.

- Digitalized lives -

This group also carried 40 percent of their age cohort's obese weight, and filed 36 percent of personal-injury insurance claims.

The real-life laboratory giving rise to these numbers is a long-term evaluation that has tracked 1,000 New Zealanders from cradle to the cusp of middle age.

A fifth of 38-year-olds in New Zealand account for four-fifths of criminal convictions and two-thirds of welfare dependence in their age group, the study found.

All the participants in the so-called Dunedin Study -- representing the full socio-economic spectrum of New Zealand society -- were born in the city of that name between April 1972 and March 1973.

Follow-up tests and evaluations were performed at two-year intervals up to age 15, and at three- or four-year intervals after that.

The outcomes were matched against comprehensive government and health records.

"We know every location they've lived, every name they've used," said Terrie Moffitt, a professor at Duke who also took part in the study.

"The digitalization of people's lives allows us to quantify precisely how much a person costs society," he added, noting that many experts were skeptical that the Dunedin data could be accurately linked up with public records.

The Big Brother overtones of the study should not be used to single out the "costly" members of society for ridicule or blame, the authors cautioned.

Being able to identify children struggling early in life should instead be seen as an opportunity to intervene and "change their trajectories -- for everyone's benefits," Moffitt said.

The researchers argued that New Zealand is a good "laboratory" because it has income gaps similar to those in the United States and Britain, and spends a comparable amount on health care.