A study of inmates at Christchurch women's prison has found nearly every woman interviewed had a history of multiple traumatic brain injuries.
The study was cited by Dr Ian Lambie, the chief science advisor to the justice sector, who has published a discussion paper looking at the prevalence of head injuries in people who end up in courts and prisons.
People with brain injuries are over-represented in the justice system, and Dr Lambie said more could be done to deal with the problem.
The 2017 study by the University of Canterbury talked to 38 inmates from all security levels at Christchurch Women's Prison.
All but two came back as having a history of more than one traumatic brain injury, a quarter of which were inflicted by a partner or parent.
Dr Lambie said it was a startling study.
"That's a very sad and tragic stat, and what I think it's important to emphasise is there's not a person that I've worked with who's been involved in the justice [system], both adults and also young people, who aren't victims themselves."
Dr Lambie said the fact someone had a brain injury or developmental issue did not excuse their offending, but offered a better understanding of why someone offended and how to prevent it in the first place.
He said it was not always obvious an that offender had a brain injury.
"The thing about it is if someone breaks a leg or breaks an arm and they're wandering down the street, they might be on crutches or in a cast or a sling and you know that person has broken an arm or a leg.
"The thing about concussion or traumatic brain injury, people don't walk around with a label on their head saying I've got traumatic brain injury.
"It often goes undetected, unseen."
Sally Kedge is the director of Talking Trouble Aotearoa New Zealand, an organisation that offers speech therapy and support to people going through the justice system.
She sometimes sits down with people as they go through a court case, translating legal jargon into something they can understand.
"Sometimes we sit there and wonder what is being spoken about," she said.
"Even with a high level of education and a very good vocabulary, it can be very confusing."
Dr Lambie said a common perception in court was that defendants looked shifty, mumbled, lacked emotion, and would not look anyone in the eye.
He said those were also signs of a brain injury or developmental problem.
A key example is that of Teina Pora, who spent 21 years in prison after admitting to a murder he did not commit. He suffers from Fetal Alcohol Spectrum Disorder.
Kedge said by the time people with brain injuries were in the justice system, many opportunities to help them had been missed.
"A lot of the children or young people we see will tell us that they didn't really go to high school, and when you delve down there, they might have been to a lot of schools and been at the attention of professionals because of their behaviour.
"But what's behind that behaviour? Why are they getting into trouble and what can be done about that earlier to better equip them with skills and keep them in education?"
Dr Lambie said more needed to be done to diagnose brain injuries and drop the stigma around reporting them.
He said head injuries were underreported in the general population.
"But they're even more underdiagnosed and over-represented in the justice population," he said.
"It really paints a picture of the need for early intervention."
Dr Lambie said officials needed to ask more questions about how people with cognitive issues were dealt with in the justice sector.