Soldiers returning from Iraq and Afghanistan are more likely to commit violent crimes at home if they have witnessed traumatic events during active combat, according to the first detailed study into violent offending in the military.

An investigation of nearly 14,000 military personnel found that about 20 per cent of young servicemen under the age of 30 had at least one conviction for a violent offence compared with less than 7 per cent for young men of a similar age in the general population.

The scientists who carried out the research found a strong association between the likelihood of being a violent offender and exposure to traumatic incidents during combat. They also found a link between violent offending and alcohol misuse and post-traumatic stress disorder.

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The study found that while the overall rate of lifetime criminality among military personnel was lower than the general population, the incidence of violent offending among ex-servicemen – ranging from threats of violent to serious assaults or worse – was 11 per cent compared with 8.7 per cent among men at large.

The researchers emphasised that they have not proven that active combat causes the increased incidence of violent criminality seen among ex-military men after they return home, but they said that the link needs to be understood so that ex-servicemen can receive counselling and help.

“The findings provide information that can enable better violence risk assessment in serving and ex-serving military personnel,” said Deidre MacManus of King’s College London, the lead author of the study published in The Lancet.

“They draw attention to the role of mental health problems and the potential effect that appropriate management of alcohol misuse, post-traumatic stress disorder… and aggressive behaviour could have in reducing the risk of violence,” Dr MacManus said.

The study compared military records of veterans of the Iraq and Afghanistan conflicts, provided by the Ministry of Defence, with criminal records held on the Ministry of Justice’s Police National Computer. Of the 13,856 randomly selected serving and ex-serving personnel, 12,359 were men.

Although criminal behaviour in general over a person’s lifetime was lower among the military compared with the general male population, the increased incidence of violent offending was significantly higher, Dr MacManus said.

“There has been a lot of media coverage and public debate about violence committed by veterans of the conflicts in Iraq and Afghanistan. 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,” Dr MacManus said.

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“Serving in a combat role and traumatic experiences on deployment also increased the risk of violent behaviour….There was a three-fold increase in the risk of violent offending among men who served in a combat role,” she said.

Professor Sir Simon Wessely, a co-author of the study, said that men who had a record of violence before joining the military were more likely to be violent offenders after they had completed their term of service and that alcohol misuse often played a significant role.

“The problems are often when people have left the military. They leave early and they re-offend. These are issues that are not just health issues, they cross the health and social-care divide. We already know that alcohol plays a role in most disciplinary offences in the military,” Professor Wessely said.

“It’s the nature of the profession [to be aggressive]. Some people with aggressive dispositions make very good soldiers. You meet a lot of people in the armed forces who you are rather glad are in the military – it does them a lot of good. Equally, there are some who do not do well and go on to become violent offenders,” he said.

Over the past 10 years a number of studies have found links between active combat and adverse effects on the mental health of service personnel. In addition to post-traumatic stress disorder, ex-servicemen are more likely to suffer anxiety, depression and alcohol misuse compared to men of the same age in the general population, the study says.

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 two per cent 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.”