Head injury can dramatically increase the chances of someone committing violent crime, a study has found.
In a large group of 22,914 traumatic brain injury victims, almost 9% went on to commit acts of violence after diagnosis.
Compared with the general population, people with brain injuries were three times more likely to perpetrate violent crimes.
The British and Swedish researchers who carried out the study defined violent criminals as those convicted of homicide, assault, robbery, arson, sexual offences, or illegal threats or intimidation.
Epilepsy was also investigated as previous studies have suggested it can increase the risk of violence.
However, after adjusting for family background, the scientists found no significant association between a diagnosis of epilepsy and violent crime.
To carry out the investigation, the authors matched every case of epilepsy and traumatic brain injury recorded in Sweden between 1973 and 2009 with 10 people free of these conditions. The study was the largest of its kind ever carried out with both the brain injury and epilepsy groups totalling more than 22,000 people. It found that 8.8% of the brain injury group were convicted of violent crime after diagnosis compared with just 3% of the general population.
The findings were published in the online journal Public Library of Science Medicine.
Certain types of epilepsy and brain injury were more likely to be associated with violence than others, the study showed.
Author Dr Seena Fazel, from Oxford University, and colleagues wrote: "Health services may consider violence risk assessment and management worthwhile in certain high-risk groups of individuals with epilepsy, particularly if they have violent histories. For traumatic brain injury, absolute and relative risks more clearly suggest that there are certain groups of patients who would benefit from violence risk assessment.
"As current guidelines for the assessment of brain injury make no recommendations in relation to the assessment or investigation of violence risk, our findings suggest that these may need review, at least for some groups of patients with traumatic head injury, particularly if they abuse illegal drugs or alcohol."