Explaining Rare Acts of Violence: The Limits of Evidence From Population Research

Nielssen and colleagues (8) identified seven population studies in six countries (none in the United States) that reported stranger-homicides by people with schizophrenia or other psychotic illness. Seventy-eight of these incidents were counted across all studies and years. Assuming that .5% of the population has schizophrenia, the researchers estimated that 1 in 70,000 schizophrenia patients per year commit a stranger-homicide.

These authors' composite statistical profile of persons with psychotic illness who killed strangers shows that 95% of the perpetrators were male and that their average age was 32 years. Sixty-two percent had never been admitted to a psychiatric hospital. Sixty-four percent had no prior treatment with antipsychotic medication, and only 12% were taking an antipsychotic medication at the time of the homicide. Seventy-four percent were not in contact with mental health services in the month before the homicide. Seventy-nine percent were unemployed, and 17% were homeless. Fourteen percent had low intelligence. Nineteen percent had a history of child abuse. Thirty-eight percent had a history consistent with childhood conduct disorder, and about the same proportion (40%) displayed repeated antisocial conduct as adults. Nineteen percent had a prior violent offense (8).

It is difficult to know what to make of these findings. Although not uninformative, such descriptive data in themselves are hardly amenable to causal modeling of homicide, to say nothing of studying interventions to avert it. For every homicide perpetrator with schizophrenia who fits the profile of risk factors, there are tens of thousands of people with the same risk factors who will never commit a homicide. Where does that leave us?

Nielssen and colleagues (8) offer a modest summation of their study's implications, which surely seems sensible even if the data cannot prove them right: “Measures that ensure earlier treatment of psychosis and continued treatment in the community would be likely to prevent homicides of both strangers and family members. However, the extreme rarity of these events means that identification of individual patients who might kill a stranger is not possible.”

With respect to prevention, the data provide general hints at best. Reports from the National Confidential Inquiry Into Suicide and Homicide by People With Mental Illness (9) in the United Kingdom suggest, for example, that reducing co-occurring substance abuse and ensuring regular outreach to improve treatment adherence among patients with a history of violence might prevent future violence. But we know that already, and it does not prevent a Tucson. It does not stop a mass shooting by a disturbed individual untouched by the mental health service system who can legally buy a handgun with a high-capacity magazine that fires 31 bullets in 15 seconds.

Either of two factors can thwart a public health intervention study: a small effect size or a rare outcome event. Studies of how to prevent gun violence by people with mental illness would face both of these challenges at once.

This brings up the question of triangulating evidence of common things to inform understanding of similar but far less common things. Can studies of minor aggressive acts by psychiatric patients tell us anything useful about the one-in-millions mass homicide perpetrator? Maybe, or maybe not. We don't know—at least not on the basis of the evidence we have now. By analogy, studies of hypertension treatment alone would never have produced the medical knowledge that beta blockers can lower death risk in heart attack victims. To fully understand and explain such events, we need to study them directly and describe them.

But even then, description is not the same as prediction. Profiles of mass killers suggest that they tend to be troubled young men. This does not tell us which one of the very large population of troubled young men in the world will become the next mass killer; the vast majority of them will not. And even if we could statistically predict such violent events, it would not necessarily mean that we could explain them. History of previous violent behavior is a relatively strong statistical predictor of any future violent behavior. It does not explain violence any more than a history of previous cancer explains why people get cancer.

When asked to explain individual acts of horrific violence committed by people who appear to have been mentally ill, perhaps the best that a researcher can do—at least for now—is to restate what we know about any and all violence risk in populations, and to say it in several ways: absolute risk, relative risk, and attributable risk. The “absolute risk” message is that the vast majority of people with mental illness in the community are not violent. The “relative risk” message is that people with serious mental illness are, indeed, somewhat more likely to commit violent acts than people who are not mentally ill. And the “attributable risk” message is that violence is a societal problem caused largely by other things besides mental illness (ready availability of guns, for example) (10.)

That said, I am surely not suggesting that we are ultimately powerless to prevent rare and terrible things (such as 9/11) simply because they almost never occur and we cannot explain them by using population databases. Such a position would amount to irresponsible nihilism. From a public health point of view, we face some problems that almost inherently defy explanation and predictability, yet we prevent them anyway. We act on reasonable assumptions despite idiosyncratic and fragmentary evidence, because we have no other choice.