While individuals diagnosed with mental illness only account for 4 percent of all violent crime in the United States, a higher portion of perpetrators of mass homicides are mentally ill in comparison to perpetrators of other types of violence.
Correlation is not causation — even when a person who commits mass violence is found to have a diagnosable mental illness, it is not clear that mental illness was the precipitating factor in the crime. Having a psychiatric diagnosis is neither necessary nor sufficient a risk factor for committing an act of mass violence.
Violent behavior associated with serious mental illness (i.e., a DSM diagnosis of schizophrenia spectrum disorder, bipolar disorder or major depression) alone is about 4 percent. Meaning 96 percent of violent events would still occur because they are caused by factors other than mental illness.