Mental illness is usually not linked to crime, according to a recent study.

Researchers found that only 7.5 percent of crimes committed were directly related to symptoms of mental illness.

In the United States, more than 1.2 million people with mental illness are incarcerated in jails or prisons, according to the federal Bureau of Justice Statistics. People with mental illnesses also are on probation or parole at two to four times the rate for the general population.

"When we hear about crimes committed by people with mental illness, they tend to be big headline-making crimes so they get stuck in people's heads," lead researcher Jillian Peterson, PhD, said in a statement. "The vast majority of people with mental illness are not violent, not criminal and not dangerous."

For the study, researchers analyzed 429 crimes committed by 143 offenders with three major types of mental illness and found that 3 percent of their crimes were directly related to the symptoms of major depression, 4 percent to symptoms of schizophrenia disorders and 10 percent to symptoms of bipolar disorder.

Study participants completed a two-hour interview about their criminal history and mental health symptoms, covering an average of 15 years.

Researchers said they did not find any predictable patterns linking criminal conduct and mental health symptoms, covering an average of 15 years. Researchers also reviewed heir criminal history and social worker files to help rate crimes based on their association with symptoms of schizophrenia disorders, bipolar disorder or major depression.

"Is there a small group of people with mental illness committing crimes again and again because of their symptoms? We didn't find that in this study," Peterson said.

Two-thirds of the offenders who had committed crimes directly related to their mental illness symptoms also had committed unrelated crimes for other reasons, such as poverty, unemployment, homelessness and substance abuse, according to the research.

The findings were recently published in the American Psychological Association's Journal Law and Human Behavior.