There was an interesting Harper's article which talked about the opposite question: “Why do large-scale disasters produce such mentally healthy conditions?”

The contention was that our leaders expect always chaos in the streets, but usually the atmosphere is one of mutual aid, and there is a distinct drop in destructive or neurotic behavior. Totally by coincidence, this was on the newsstands when Katrina hit.

I was impressed by the article at the time, but it sounds like you're in a position to know whether it's backed up by the facts?