One one hand, we have a growing body of research saying that almost nothing anybody does is rational, and that most "rational explanations" that people deliver are post-hoc rationalizations of something they did on a whim. The popular interpretation is that this represents evidence against free will, but that may be a little overblown. On the other hand, there is a parallel cultural trend of people arguing that suicide is rational, and that many suicidal patients are that way because they sat down and added up the utility of the various options. I guess my point is, when obesity drops to zero on the day that everybody realizes that it isn't rational to be fat, I'll start considering the possibility that people could "decide" to kill themselves.