Certain mental ills may be tied to violence.

Although most people with psychiatric disorders don't attack others or pursue lives of crime, the relationship of mental illness to violence remains unclear. A long-term study in New Zealand now links elevated violence rates in young adults to the presence of at least one of three psychiatric ailments--alcohol dependence, marijuana dependence, and a range of psychotic experiences and beliefs called schizophrenia-spectrum disorder.

"Our study suggests that a significant proportion of the violence that frightens and injures the general public may be attributed to young adults who are prone to [these disorders], many of whom have not been hospitalized or treated," says a team led by Louise Arseneault and Terrie E. Moffitt of the Institute of Psychiatry in London. No other psychiatric disorders showed this link in the recent study.

Earlier delinquency and drug problems appear to have contributed both to the three disorders and to violence in young adults, the scientists say in the October ARCHIVES OF GENERAL PSYCHIATRY.

The researchers studied 961 men and women, all age 21, who were born in New Zealand and have been interviewed at regular intervals since age 5. Self-reports and official conviction records identified acts of physical violence--such as an attack on someone with a weapon, robbery, rape, and gang fighting--committed by each participant in the year before the most recent interview.

While people with alcohol dependence, marijuana dependence, schizophrenia-spectrum disorder, or more than one of these diagnoses constituted one-fifth of the sample, they were responsible for more than half of all self-reported and recorded violent crimes, the researchers report.

People with schizophrenia-spectrum disorder displayed an increased risk of violence that was independent of the effects of substance abuse. The data indicate that a tendency to perceive excessive threat in their surroundings contributed to their violent acts, Arseneault and her coworkers report.

The New Zealand study is the most comprehensive communitywide probe of mental disorders and violence, says psychologist John T. Monahan of the University of Virginia in Charlottesville.

However, violent behavior occurs more often in the United States than in New Zealand, Monahan notes. "More of the violence in New Zealand may be attributable to mental illness than in a place like New York City," he says.

Other studies, which focus on people discharged from U.S. psychiatric hospitals, find no increased violence among those with schizophrenia or with violent delusions or fantasies.

Certain personality disturbances may instead promote violence, says psychologist Jeffrey G. Johnson of Columbia University. Traits such as suspiciousness of others and lack of empathy markedly contributed to violence in young adults in the general population who were tracked for 8 years in upstate New York, Johnson and his coworkers report in the September AMERICAN JOURNAL OF PSYCHIATRY.