People who commit violent offences are more likely to suffer from a personality disorder than a mental one, a local forensic psychologist said yesterday.

"Public perception is that there is a strong relationship between mental illness and violent behaviour," said Dr. Liam Ennis, a private clinician in Edmonton who is often called to testify in sentencing hearings.

"It's grossly overstated, especially compared to the relationship between violence and substance abuse, personality disorders and general anti-social inclinations."

Ennis said people often jump to incorrect assumptions about mental illness such as depression and schizophrenia - which can be treated with medications - versus personalities disorders such as antisocial personality disorder and psychopathy - which are difficult to treat, require extensive counselling and are often resistant to medication.

It's a stigma that engenders fear of the mentally ill, many of whom live on city streets and are homeless.

"Even if a mentally ill person were to become violent, they would rarely attack a stranger. It's mostly family or friends who have somehow disrupted routine."