New research has found 9 out of 10 kids at a juvenile detention centre have at least one form of severe brain impairment.

The potentially ground-breaking study from The Telethon Kids Institute found that 89 per cent of the 99 youths assessed at the Banksia Hill detention centre had at least one form of severe neurodevelopmental impairment that makes them more likely to have problems with the law.

The study also, for the first time, assessed the prevalence of Foetal Alcohol Spectrum Disorder (caused by prenatal exposure to alcohol) among youths in custody in Australia.

For many of the study’s participants, it was the first time they had received a comprehensive assessment, despite many having a history with child protection services and the justice system.

“These are missed opportunities for earlier diagnosis and intervention, which may have prevented or mitigated their involvement with justice services,” the report found.

“We've failed these young people actually, society's failed these young people,” said lead researcher Professor Carol Bower.

The researchers says their study should help change the way troubled youths are dealt with in courts and in detention.

Thirty-six per cent of the youths were found to have FASD - almost twice the rate identified in a study of children in Western Australia's Fitzroy Valley.

Sixty-five per cent showed at least three forms of severe neurodevelopmental impairment, while 23 per cent had five or more.

The impairments have previously been linked to problems with memory, cognition, motor skills, social skills, attention and executive function.

This means that many cannot effectively relate cause and effect or plan things.

“Impairment in domains such as language, executive function, memory and cognition, may contribute to offending behaviours and/or difficulties in negotiating all aspects of the justice system,” the report stated.

Almost a quarter of the young people assessed were found to have an IQ score of less than or equal to 70, which is the cut off below which adults are deemed unfit to plead in court.

Professor Bower said the study could have broad implications for the way health, education, justice, child protection and other agencies manage troubled young people.

“There's no reason to think there isn't a similar situation across Australia in the justice system,” Professor Bower said.

“What this study does is gives us evidence to inform system change for better outcomes for the young person and their communities.”

Researcher Raewyn Mutch said many were simply written off as ‘bad kids’, when they actually have severe impairment.

“Your difficult behaviour comes from an underlying organic brain injury, it's not wilful. It's behaviour by accident of how your brain works, not by your wilful choice,” Dr Mutch told reporters.

“Our study provides strong science to support spending the money differently, so these young people can be rehabilitated as early as possible and that diminishes the other costs, such as grief and loss, whether you are the victim or the one who carried out the crime.”

The research was conducted with the support of the WA Department of Justice and the Department of Communities, which say they will use its results to help inform specific rehabilitation plans for young detainees.

The authorities also want to work on ways to identify brain impairments among troubled youths earlier.