Have by no means read all the replies to this post, but just a note on the debate. From what I'v observed the definition of IBS is largely arbitrary. In the broadest sense, it has been applied to any kind of chronic digestive disturbance that has no other explanation.
For my own part, I absolutely have my own IBS due to psychological problems, ie the brain-gut element is acutely valid in my case. I have often had empirical evidence of this over the years in that more emotional harmony in my life led to things going smoothly in the bathroom area as well. Besides, I had symptoms indicative of disturbed seratonin levels well before IBS struck and when I still had no digestive issues whatsoever. I think the problem may often lie with the nature of the umbrella term that is IBS and its ambiguous definition.
-------------------- Susie, born in 1985,
(pseudo-)D and bloating April 2007-December 2010, now stable