Research on the brain is still in its infancy. Do you think we will ever know enough about the brain to prove that certain psychiatric diagnoses have a direct biological cause?

I'd be willing to bet everything that whenever it happens, whatever we find out about the brain and mental suffering is not going to map, at all, onto the DSM categories. Let's say we can elucidate the entire structure of a given kind of mental suffering. We're not going to be able to say, "here's Major Depressive Disorder, and here's what it looks like in the brain." If there's any success, it will involve a whole remapping of the terrain of mental disorders. And psychiatry may very likely take very small findings and trump them up into something they aren't. But the most honest outcome would be to go back to the old days and just look at symptoms. They might get good at elucidating the circuitry of fear or anxiety or these kinds of things.

What is the difference between a disorder and distress that is a normal occurrence in our lives?

That distinction is made by a clinician, whether it's a family doctor or a psychiatrist or whoever. But nobody knows exactly how to make that determination. There are no established thresholds. Even if you could imagine how that would work, it would have to be a subjective analysis of the extent to which the person's functioning is impaired. How are you going to measure that? Doctors are supposed to measure "clinical significance." What's that? For many people, the fact that someone shows up in their office is clinical significance. I'm not going to say that's wrong, but it's not scientific. And there's a conflict of interest -- if I don't determine clinical significance, I don't get paid.

You say one of the issues with taking these categories too seriously is that it eliminates the moral aspect behind certain behaviors.

It's our characteristic way of chalking up what we think is "evil" to what we think of as mental disease. Our gut reaction is always "that was really sick. Those guys in Boston -- they were really sick." But how do we know? Unless you decide in advance that anybody who does anything heinous is sick. This society is very wary of using the term "evil." But I firmly believe there is such a thing as evil. It's circular -- thinking that anybody who commits suicide is depressed; anybody who goes into a school with a loaded gun and shoots people must have a mental illness. There's a certain kind of comfort in that, but there's no indication for it, particularly because we don't know what mental illness is.

How do diagnoses affect people?

One of the overlooked ways is that diagnoses can change people's lives for the better. Asperger's Syndrome is probably the most successful psychiatric disorder ever in this respect. It created a community. It gave people whose primary symptom was isolation a way to belong and provided resources to those who were diagnosed. It can also have bad effects. A depression diagnosis gives people an identity formed around having a disease that we know doesn't exist, and how that can divert resources from where they might be needed. Imagine how much less depression there would be if people weren't worried about tuition, health care, and retirement. Those are all things that aren't provided by Prozac.