Young men who have served in the military are much more likely to commit violent crimes than those who haven't, a study has shown.
Men who had seen combat in Iraq and Afghanistan were 53% more likely to commit a violent offence than comrades given non-combat roles.
Those with multiple experiences of combat had a 70% to 80% greater risk of committing acts of violence.
Of around 3,000 military men under the age of 30, more than a fifth had a conviction for violent offences, compared with 6.7% of their civvie street peers.
Overall, 17% of the men had criminal records, and 11% had committed violent offences.
Of the 2,728 aged 30 and younger, 20.6% had a criminal record for violence.
There were strong links between combat experience, post-deployment alcohol misuse, traumatic stress and violence.
The findings, released on the 10th anniversary of the Iraq War, are published in a special issue of The Lancet medical journal.
In the biggest study of its kind ever undertaken, researchers were given access to police records on almost 14,000 randomly selected men and women who were active or former members of the armed forces, mostly the Army.
Participants provided information about their experiences before and after joining the military and underwent psychological tests.
A search of the Police National Computer was made for any convictions, cautions or warnings relating to the study population.
Study leader Dr Deirdre MacManus, from King's College London, said: "Our study, which used official criminal records, found that violent offending was most common among young men from the lower ranks of the Army and was strongly associated with a history of violent offending before joining the military.
"Serving in a combat role and traumatic experiences on deployment also increased the risk of violent behaviour."
Violent offences covered a broad range of acts, from verbal harassment to homicide. They did not include incidents of domestic violence.
A strong association was seen between violent offending and high levels of self-reported aggression.
Professor Sir Simon Wessely, co-director of the Centre for Military Health Research at King's College London, who co-authored the study, admitted that an aggressive streak can be useful in a soldier.
"Some people with aggressive dispositions make very good soldiers, that's the nature of the game," he told a press conference in London."
But he stressed that the vast majority of soldiers returning from tours of duty in combat zones never got into trouble.
"Not every single person who joins the armed forces is Sir Lancelot or Sir Galahad," said the professor. "We are suggesting there is a problem that needs to be looked at, but just as with post traumatic stress disorder this is not a common
outcome in military populations. Overall you must remember that of those who serve in combat, 94% of who come back will not offend."
Screening within the armed forces to identify at-risk individuals would not work, he argued. For every correct prediction there were likely to be five that were wrong.
However he said his group had started conducting random mental health checks on several thousand members of the armed forces returning from active duty. In the US, this is a routine procedure.
Commenting on the report, a Ministry of Defence spokesman said: "We are committed to supporting members of our Armed Forces and their families as they return to civilian life post deployment.
"That is why we funded this research and have comprehensive mental health support available before, during and after operations. We also ensure that all personnel go through a thorough period of decompression to help make this adjustment.
"This report recognises that the vast majority of service personnel make this adjustment successfully and are not more likely to commit a violent offence post deployment - there is only an increased risk of 2% when compared to the general population.
"However, any violent offence is unacceptable and will not be tolerated by our Armed Forces. If a member of our Armed Forces or their family experiences violence there is a wide range of support and help available to them."