NEW YORK -- About half of all 16- to 18-year-olds coming into New York City's jails say they had a traumatic brain injury before being incarcerated, most caused by assaults, according to a new study that's the latest in a growing body of research documenting head trauma among young offenders.

And in the United Kingdom, a national campaign on the issue has resulted in a commission that found almost two-thirds of young inmates suffered from head trauma, which University of Exeter researchers found in 2010 is associated with earlier, repeated and more time spent in custody.

Experts say the findings, published this week in The Journal of Adolescent Health, could lead to better training for correction officers on how to deal with the possible symptoms of such trauma, which include problems with impulse control and decision-making.

"You need to train the correction officers to understand brain injuries so that when somebody may be acting rude or answering back or forgetting what they're supposed to do, it's not a sign of maladaptive misbehaviour or disrespect, it's a sign of a brain injury," said Wayne Gordon, a brain injury expert at New York's Mount Sinai Hospital.

The peer-reviewed study was based on medical brain injury questionnaires given to 300 boys and 84 girls inside America's second-largest jail system in 2012.

The study found nearly 50 per cent of both boys and girls reported traumatic brain injuries that resulted in a loss of consciousness, amnesia or both. And they said 55 per cent of those injuries were caused by assaults.

Previous studies show the rate of traumatic brain injury among adolescents who aren't incarcerated is about 15 to 30 per cent, said Dr. Homer Venters, an assistant health commissioner in New York City and one of the study's authors.

Brain injuries are often undiagnosed or misdiagnosed because people with them don't necessarily show obvious, immediate signs of injury. But research about them has increased in recent years, as combat veterans and children who play contact sports have displayed symptoms, experts said.

A growing body of research shows that inmates whose brains have been jolted by trauma are linked to higher rates of breaking jailhouse rules, substance abuse and greater difficulty re-entering society after detention, said John D. Corrigan, a professor in the Department of Physical Medicine and Rehabilitation at Ohio State University and a national expert on head injuries.

An estimated 60 per cent of adult prisoners have a brain injury, according to a study of prisoners in South Carolina. Not all correction departments screen inmates for the injury -- a practice public health officials say should change.