There is a growing body of research aimed at determining which of these two categories a syndrome falls into.
Dr. David Clarke, a gastroenterologist who is considered a leading expert on stress-induced syndromes, or "mind-body" syndromes, says this in regard to ulcerative colitis (UC):
"An answer to [whether or not UC is caused by stress] will await the results of much more research. We do have research results that confirm an interaction between the mind and the immune system mediated by the neurologic system. The existence of this interaction came as a big surprise and resulted in the research field known as psychoneuroimmunology (PNI). (For information about the origin of this field, see my blog here: http://www.stressillness.com/blog/?p=1471