See Slate's complete coverage of the Gabrielle Giffords shooting and arrest of Jared Lee Loughner.
Shortly after Jared Lee Loughner had been identified as the alleged shooter of Arizona Rep. Gabrielle Giffords, online sleuths turned up pages of rambling text and videos he had created. A wave of amateur diagnoses soon followed, most of which concluded that Loughner was not so much a political extremist as a man suffering from "paranoid schizophrenia."
For many, the investigation will stop there. No need to explore personal motives, out-of-control grievances or distorted political anger. The mere mention of mental illness is explanation enough. This presumed link between psychiatric disorders and violence has become so entrenched in the public consciousness that the entire weight of the medical evidence is unable to shift it. Severe mental illness, on its own, is not an explanation for violence, but don't expect to hear that from the media in the coming weeks.
Seena Fazel is an Oxford University psychiatrist who has led the most extensive scientific studies to date of the links between violence and two of the most serious psychiatric diagnoses—schizophrenia and bipolar disorder, either of which can lead to delusions, hallucinations, or some other loss of contact with reality. Rather than looking at individual cases, or even single studies, Fazel's team analyzed all the scientific findings they could find. As a result, they can say with confidence that psychiatric diagnoses tell us next to nothing about someone's propensity or motive for violence.
A 2009 analysis of nearly 20,000 individuals concluded that increased risk of violence was associated with drug and alcohol problems, regardless of whether the person had schizophrenia. Two similar analyses on bipolar patients showed, along similar lines, that the risk of violent crime is fractionally increased by the illness, while it goes up substantially among those who are dependent on intoxicating substances. In other words, it's likely that some of the people in your local bar are at greater risk of committing murder than your average person with mental illness.
Of course, like the rest of the population, some people with mental illness do become violent, and some may be riskier when they're experiencing delusions and hallucinations. But these infrequent cases do not make "schizophrenia" or "bipolar" a helpful general-purpose explanation for criminal behavior. If that doesn't make sense to you, here's an analogy: Soccer hooligans are much more likely to be violent when they attend a match, but if you tell me that your friend has gone to a soccer match, I'll know nothing about how violent a person he is. Similarly, if you tell me your friend punched someone, the fact that he goes to soccer matches tells me nothing about what caused the confrontation. This puts recent speculation about the Arizona suspect in a distinctly different light: If you found evidence on the Web that Jared Lee Loughner or some other suspected killer was obsessed with soccer or football or hockey and suggested it might be an explanation for his crime, you'd be laughed at. But do the same with "schizophrenia" and people nod in solemn agreement. This is despite the fact that your chance of being murdered by a stranger with schizophrenia is so vanishingly small that a recent study of four Western countries put the figure at one in 14.3 million. To put it in perspective, statistics show you are about three times more likely to be killed by a lightning strike.
The fact that mental illness is so often used to explain violent acts despite the evidence to the contrary almost certainly flows from how such cases are handled in the media. Numerous studies show that crimes by people with psychiatric problems are over-reported, usually with gross inaccuracies that give a false impression of risk. With this constant misrepresentation, it's not surprising that the public sees mental illness as an easy explanation for heartbreaking events. We haven't yet learned all the details of the tragic shooting in Arizona, but I suspect mental illness will be falsely accused many times over.