Young men who have served in the British military are about three times more likely than civilians to have committed a violent offence, researchers reported Friday in a study that explores the roots of such behaviour.

The research found that merely being sent to Iraq or Afghanistan made no difference in rates of violent crime later on. Instead, a key predictor was violent behaviour before enlisting. 

Combat duty also raised the risk, as did witnessing traumatic events during deployment or misusing alcohol afterward.

Still, the vast majority - 94 perc ent - of British military staff who return home after serving in a combat zone don't commit any crimes, researchers told reporters at a briefing.

The study found little difference in the lifetime rates of violent offenses between military personnel and civilian populations at age 46 - 11 per cent versus almost 9 per cent. 

Among younger men, however, being in the military seemed to make a difference: Nearly 21 percent of the military group under age 30 had a conviction for a violent offense in their lifetime compared to fewer than 7 per cent of similarly aged men in the general population, according to British crime statistics.

"The problem is that some of the qualities you want in a soldier are the same ones that get people arrested for violent behaviour," said Walter Busuttil, director of medical services for Combat Stress, a British veterans' charity that was not part of the study. 

Busuttil said many of those recruited into the army are from disadvantaged backgrounds where violence is more common.

The research was published online Friday in the medical journal Lancet. Researchers at the Institute of Psychiatry at King's College London looked at data from more than 13,800 U.K. military personnel and veterans and compared that to records of violent crimes ranging from verbal threats to assaults and homicides. 

Some people were followed for up to seven years. Nearly 1500 women were included, though they were mostly in noncombat roles.

Deirdre MacManus, the study's lead author, said combat experience seemed to matter when they compared violent crime rates among military personnel. 

"Being deployed in itself wasn't a risk factor for violent offenses but being exposed to multiple traumas, like seeing someone get shot, increased the risk by 70 to 80 percent," she said, compared to someone who hadn't witnessed such a harrowing ordeal.

Researchers said other studies have made similar findings.

Britain currently has some 5000 soldiers in the NATO-led mission fighting in Afghanistan and it is the second-largest foreign contingent after the US It withdrew its soldiers from Iraq in 2009 after six years. 

The UK Ministry of Defence has been under pressure to develop more mental health programs for veterans after reports of returning servicemen committing crimes, like the 2012 case of an ex-soldier in Leeds jailed for shooting his landlady after fighting in Afghanistan.

He had been diagnosed with post-traumatic stress disorder but had not been monitored or treated.

In the US, there have been numerous cases of veterans committing violent crimes, including a Marine charged with killing six people in California last year. Soldiers from a single Army unit in Colorado killed 11 people over a few years after their return home. 

An Army report in 2009 placed part of the blame on the psychological trauma of fierce combat in Iraq. And this week, a US Senate panel heard women in the military describe sexual assaults by fellow soldiers.

American researchers said it is possible the same links the British study found between fighting in a war and violent crime exist in the US but that there isn't enough data yet.

"For some soldiers, it's hard to stop being a warrior," said Brett Litz, a psychology professor at Boston University who studies veterans' issues. 

"What happens during a war may be a prescription for a small percentage of men to get into trouble," he said. "They may find it very difficult to switch out of a wartime mindset."

But given the differences between Britain and the U.S., Litz said it was impossible to predict what effects would be seen in the United States. "Maybe the economy is better here, maybe the (department of defense) does a better job with transition, maybe (the) culture is different especially with respect to alcohol," Litz said.

"There will be a lot of returning soldiers who have to be reintegrated and unfortunately there is no quick fix," he said.

- AP