Mass killers fit no single profile and no pattern of insanity. Many, if not most, had never been diagnosed with a serious psychiatric disorder. Background checks can prevent someone with a diagnosis of mental illness from acquiring a gun, but psychologists cite a wide divide between a clinical diagnosis and the type of emotional disturbance that precedes many mass killings, the New York Times reports. Mental illness is not a useful means to predict violence. About half of all Americans experience mental health issues at some point in their lives, and the vast majority of people with mental illness do not kill. “Do you or do you not have a mental health diagnosis?” said Jillian Peterson of the Violence Project, which has compiled a database of mass shootings from 1966 on and studied perpetrators in depth. “In many cases, it doesn’t really matter. It’s not the main driver.” Experts have come to focus on warning signs that occur whether or not mental illness is present, including marked changes in behavior, demeanor or appearance, uncharacteristic fights or arguments, and telling others of plans for violence, a phenomenon known as “leakage.” The warning signs approach can work even when the mental health system does not, and it sidesteps the complaint that blaming mass shootings on mental illness increases negative attitudes and stigma toward those who suffer from it. University of Virginia education Prof. Dewey Cornell cites a high school freshman in West Paducah, Ky., who in 1997, brought guns to school disguised as an art project and opened fire, killing three students and wounding five. The gunman had schizophrenia and was severely delusional; his mental state had clearly worsened over time, meaning there had been opportunities to intervene. He had been bullied, had made threats to his peers and had turned in an essay about shooting a bully at school. “There were many, many warning signs and leakages and not a single student came forward and said, ‘Hey, I’m concerned,’” Cornell said. “It’s a case I use in all of my training programs to show how we can make a difference.” In Peterson’s database, more than two-thirds of the perpetrators had some history of mental health concerns, including hospitalization, counseling, psychiatric medication or a previous diagnosis. About 30 percent of the gunmen had some form of psychosis, a category of mental illness that involves difficulty determining reality, and of those, a third killed in direct response to delusions or hallucinations.