Indigenous Australians have higher prevalence rates for both incarceration and mental disorder diagnoses when compared to non-Indigenous Australians a new large-scale Griffith University study has found.

The researchers aimed to examine the overlap between mental illness and incarceration using a unique dataset consisting of information from multiple government administrative systems for a population-based birth cohort for close to 45,000 individuals born in 1990 — 6.3 per cent of whom were Indigenous Australians.

“We identified mental illness based on diagnoses received during hospital admissions and incarceration from court sentencing records,’’ said author Dr James Ogilvie from the Griffith Criminology Institute.

The study found Indigenous and non-Indigenous people experienced different types of mental illness, with Indigenous peoples more likely to experience substance use and schizophrenia disorders, while non-Indigenous people were more likely to experience anxiety and mood disorders.

Results showed about one third of prisoners had a diagnosed mental disorder, but most individuals who had been diagnosed with a mental disorder did not experience incarceration.

“Although the majority of people diagnosed with a mental illness did not experience incarceration, some mentally ill individuals are more likely to experience incarceration, including those with a substance use disorder or a serious mental illness,’’ Dr Ogilvie said.

“Incarcerated Indigenous females were the most likely to experience a diagnosed mental illness.”

The longitudinal data enabled the researchers to examine the timing of onset for mental illness and imprisonment.

“Typically, mental illness was diagnosed before individuals experience incarceration, suggesting that offending behaviour is only one elementof a larger cluster of behavioural difficulties forindividuals experiencing mental illness.

“This suggests that mental health services could play a role in preventing the onset or progression of behaviour that may lead to incarceration. In particular, effective substance use interventions delivered at critical timepoints may be an important tool to divert a proportion of mentally unwell individuals from prison.”

He said their findings reinforced that prisoners are a vulnerable group and highlight mental illness as one of the core vulnerabilities that characterise the prison population.

“In an Australian context, this may also explain why Indigenous people are overrepresented among mentally ill prisoners, given the concentration of disadvantage and vulnerability.

“The overrepresentation highlights systemic failures and the concentration of vulnerability in the Australian criminal justice system. Given Indigenous overrepresentation, significant effort should be directed toward the development of culturally appropriate mental health interventions, and particularly substance use interventions.”

The study has been published in the Journal of Australian Social Issues.