Jillian Peterson, PhD, conducted a study on 142 offenders with a serious mental illness in the United States who committed 429 crimes. The study looked specifically at three major types of mental illnesses including depression, schizophrenia and bipolar disorders and asked participants to provide a criminal history and mental health symptoms for the past 15 years. Peterson found that only 7.5 percent of the crimes committed by participants were directly related to symptoms of a mental illness. Further, there was no predictable pattern found linking criminal conduct and mental illnesses over time. Of those 7.5 percent, two-thirds of the participants had committed crimes for other reasons than those related to symptoms of a mental illness including poverty, homelessness and substance use. The study did not examine how substance use interacted with mental illnesses to influence criminal behavior.
The study also did not look at offenders with serious violent offenses because the mental health court, from which former defendants who participated in the study came from, did not adjudicate those crimes. Even though some participants did describe other violent crimes they had committed, Peterson states, “The vast majority of people with a mental illness are not violent, not criminal and not dangerous.”
These findings are similar to those made in CMHA Ontario’s discussion paper, “Violence and Mental Health: Unpacking a Complex Issue” which found that there is no causal relationship between violence and mental illnesses. The key messages of this paper are:
- Estimating the rate of violent behaviour by people with mental illnesses is complex, and a definitive causal relationship between violence and mental illnesses has not been established. There are challenges due to definitions, data gaps and technical issues related to the reliability, consistency and generalizability of available data.
- People with mental health conditions experience stigma, discrimination and social exclusion that significantly impacts on their lives, including fears that they may be violent.
- People with serious mental illnesses are more likely to be victims of violence themselves, than the general population.
To read the full paper by Peterson et al. published in Law and Human Behaviour by the American Psychological Association, visit the APA website.