A study done in the late 80s (and reinforced by later studies on a similar theme) interviewed and followed 30 participants who had all been published writers and taught writing for at least 15 years at University of Iowa's writing workshop. The study indicated that 80% of these writers had some kind of affective illness. Now, the catch with studies like this (and this one in particular) is that it was conducted by one person, and none of the "evidence" was corroborated by peers. In other words, the author of the study, a psychologist, might be said to have found what she was looking for.
This is a problem for writers, this perception (reinforced by society, writers themselves, and the entire history of Western civilization going back to Plato, for crying out loud) that writing is a form of "madness," that creativity is a gift, a divine inspiration given to us "by the gods" (thanks, Plato) and that we who write (or do any kind of creative act, really) are of necessity better at it when we're looking for our muse at the bottom of the bottle, let's say, or in the arms of our best friend's wife, or some other such nonsense. Okay, yes, this is me being polemic today. But I do get tired of this stereotype, and I would like very much to get my book that attempts to demytholgise the writing process published, so that we can all start to chip away at this misperception of writers and writing.
And if those poet-types will stop drinking absinthe and taking laudanum, or snorting coke, or downing yet another bottle of Johnny Walker Black label, it would help my cause. ;-)