Multiple interacting factors contribute to violent behavior.  Public opinion surveys suggest that many people think mental illness and violence go hand in hand. A 2006 national survey found, for example, that 60% of Americans thought that people with schizophrenia were likely to act violently toward someone else, while 32% thought that people with major depression were likely to do so.  In fact, research suggests that this public perception does not reflect reality. Most individuals with psychiatric disorders are not violent. Although a subset of people with psychiatric disorders commit assaults and violent crimes, findings have been inconsistent about how much mental illness contributes to this behavior and how much substance abuse and other factors do.  An ongoing problem in the scientific literature is that studies have used different methods to assess rates of violence — both in people with mental illness and in control groups used for comparison. Some studies rely on "self-reporting," or participants' own recollection of whether they have acted violently toward others. Such studies may underestimate rates of violence for several reasons. Participants may forget what they did in the past, or may be embarrassed about or unwilling to admit to violent behavior. Other studies have compared data from the criminal justice system, such as arrest rates among people with mental illness and those without. But these studies, by definition involving a subset of people, may also misstate rates of violence in the community. Finally, some studies have not controlled for the multiple variables beyond substance abuse that contribute to violent behavior (whether an individual is mentally ill or not), such as poverty, family history, personal adversity or stress, and so on.  The MacArthur Violence Risk Assessment Study was one of the first to address the design flaws of earlier research by using three sources of information to assess rates of violence. The investigators interviewed participants multiple times, to assess self-reported violence on an ongoing basis. They verified participants' recollections by checking with family members, case managers, or other people familiar with the participants. Finally, the researchers also checked arrest and hospitalization records.  The study found that 31% of people who had both a substance abuse disorder and a psychiatric disorder (a "dual diagnosis") committed at least one act of violence in a year, compared with 18% of people with a psychiatric disorder alone. This confirmed other research that substance abuse is a key contributor to violent behavior. But when the investigators probed further, comparing rates of violence in one area in Pittsburgh in order to control for environmental factors as well as substance use, they found no significant difference in the rates of violence among people with mental illness and other people living in the same neighborhood. In other words, after controlling for substance use, rates of violence reported in the study may reflect factors common to a particular neighborhood rather than the symptoms of a psychiatric disorder.  Several studies that have compared large numbers of people with psychiatric disorders with peers in the general population have added to the literature by carefully controlling for multiple factors that contribute to violence.  In two of the best designed studies, investigators from the University of Oxford analyzed data from a Swedish registry of hospital admissions and criminal convictions. (In Sweden, every individual has a unique personal identification number that allowed the investigators to determine how many people with mental illness were convicted of crimes and then compare them with a matched group of controls.) In separate studies, the investigators found that people with bipolar disorder or schizophrenia were more likely — to a modest but statistically significant degree — to commit assaults or other violent crimes when compared with people in the general population. Differences in the rates of violence narrowed, however, when the researchers compared patients with bipolar disorder or schizophrenia with their unaffected siblings. This suggested that shared genetic vulnerability or common elements of social environment, such as poverty and early exposure to violence, were at least partially responsible for violent behavior. However, rates of violence increased dramatically in those with a dual diagnosis (see "Rates of violence compared").
Although a subset of those with mental illness commits violent acts, there are many other overlapping factors involved, including socioeconomic factors, family history, and substance abuse.