You raise a very important question. There are no answers to it. I think the only way to answer that kind of a question is to have longitudinal studies that study people right from the get-go, from day one all the way through, for quite a few years, and have those biological, psychological, brain-imaging biomarkers collected over time to then see who develops PTSD versus who does not.
For example, if you look at people who develop PTSD, through brain-scanning we can tell that the area of the brain called the hippocampus is shrunk. It's smaller than normal. If we had longitudinal studies, we'd know whether the hippocampus was shrunk before the trauma and predisposed them, or whether it happened after the trauma. I think you need these longitudinal studies. There aren't many to be had in this area, and I think it's very important that they be done.