While some Germanwings Flight 9525 investigators sorted through debris in the Alps, others were in apartments and hospitals and doctors' offices, seeking fragments of Andreas Lubitz's life. As details leaked out—a doctor's note, a depression diagnosis, a prescription, thoughts of suicide, a broken heart, an eye problem—they seemed to add up to the story of a mentally ill young man driven to commit mass murder and suicide. They also added up to trouble for Lufthansa, Germanwings’ parent company, which, according to some experts, could now face damages in excess of an initial estimate of three hundred million dollars. The airline, the experts argue, should have known that Lubitz was not someone who should have had the lives of a hundred and fifty people in his hands.

As had happened in the cases of Jared Loughner and Adam Lanza, the attribution of the disaster to mental illness has spurred calls for more thorough mental-health screenings of people seeking access to instruments of mayhem, and for more restrictions on those diagnosed with a mental illness. But as any mental-health professional will tell you (and as many did in the wake of the crash), nearly one in three Americans meets the criteria for a mental-disorder diagnosis in any year, and more than half of us will qualify at some point in our lives. Once diagnosed, people with mental illnesses, even severe psychotic disorders like schizophrenia, do not commit violent crimes at higher rates than the rest of the population. And most people who have had suicidal thoughts do not go on to kill themselves, let alone a planeload of strangers. More intense psychological scrutiny coupled with the possibility of getting fired, as the head of an organization of German flight attendants warned, could easily backfire. "I would warn against making the crew into completely transparent people,” he said. "That would just mean that someone would not go to a doctor." Or, since diagnosis is almost entirely dependent on self-reporting, the pilot could evade mandatory diagnostic scrutiny by lying—or, as Lubitz apparently did, by confining his queries about cockpit doors and suicide methods to a search engine.

Mental disorders cannot be diagnosed reliably; every day expert clinicians square off on witness stands over the proper diagnosis (if any) of criminal defendants. Even if we could put flight crews through the psychological equivalent of an airport body scanner, the results would still provide virtually no specific information about what someone would do in the future. Mental-health workers are called upon to make predictions frequently—to help a corporation decide if a job applicant will make a good employee, say, or to help the criminal-justice system decide if a sex offender can be discharged to the community—and yet unsuitable employees still get jobs and sex offenders deemed safe still re-offend. I've been in practice for more than thirty years, and to the very small extent that I think I know what the person who just walked out my office is going to do (or not do) before I see him again, my forecast is not based primarily on his diagnosis. It's based on some combination of hunch and experience, and it is often wrong. That may be because I am a lousy clinician, but it also may be because the multitude of vectors that move someone to heinous and incomprehensible acts cannot be apprehended with the inexact instruments of diagnosis deployed by imperfect people like me.

Pilots, disaffected adolescents, rapists and murderers, you and I—all of our mental lives may be more like the weather than like billiard balls, determined by innumerable forces that amplify and distort one another in ways that make accurate predictions very difficult. The National Weather Service, despite its supercomputers and satellites, not to mention its thorough understanding of the physics of weather, is often fooled. We clinicians have neither those tools nor that knowledge. To be sure, there are mental disorders in which we know enough of the vectors to say that people who have them should not occupy certain positions. A person prone to delusions should probably not fly an airplane, and a pedophile should not teach children. But these are the exceptions rather than the rule. From all we know so far about Lubitz, he was not one of those severe cases but rather someone who was among the millions of people who once contemplated suicide and was being treated for a mood disorder. It seems that he was too normal to have been identified in advance as someone who could do something so extreme.

This is not to say that mental-health workers are useless in a situation like the Germanwings crash. Our experience and our theories don't allow us to predict who the next Andreas Lubitz will be, but they do give us the ability to assemble the fragments of his life into a coherent narrative that ends in an explosion in the Alps. We can, in other words, retrodict, and if we are good at it—if we can find a story that incorporates the events and fits their magnitude—we can bring the consolations of narrative to otherwise inexplicable occurrences. Nothing that we come up with is likely to prevent the next disaster, but it will at least provide an explanation, and with it some respite from the discovery, which we make again and again, that life can turn on a dime, and that when it does it is often for no reason at all, or for reasons that cannot be grasped.

It is comforting to think that Lubitz was mentally ill. That would mean, among other things, that wise doctors could have figured out what the problem was and have fixed it, or at least they could identify it in other potential Lubitzes. But it is unlikely that even the best psychiatric evaluation would have prevented the Germanwings disaster. The depravity of the human heart cannot be contained in a vessel as flimsy as a psychiatric diagnosis.