PTSD is a weird diagnosis. It's the only major diagnosis where we key off of a causal event. All the other diagnoses are just phenomena, they happen, and you don't have to say why. In fact, that's not part of the diagnostic criteria. You don't, in making a diagnosis of schizophrenia for example, you don't key off some sort of an event, like poor parental toilet training or one of these old-fashioned ideas, they're just not part of the diagnostic criteria.
So it makes it very difficult to think about and conceptualize PTSD, because you have to include the exposure to the traumatic event in trying to assess the phenomenon. So if somebody has not had the same kind of exposure, how do you conceptualize it? Is it the same thing, is it something else?
That leads to the importance of differentiating psychiatric illness from distress. Most of the population, after an event of the magnitude of 9/11, experienced some distress. At what point do you decide that it becomes illness? And who is a candidate to become ill after an event based on the level of exposure to that event, or the kind of event it was? These are issues the field hasn't completely sorted out yet.