Recent studies indicate that the previously reported increase in annual incidence in high-incidence areas for inflammatory bowel disease has ceased. An increase in formerly low-incidence areas is still evident. A northndash;south gradient is also still present in both the United States and western Europe but is less pronounced than previously thought. There is a female predominance in Crohn's disease and a male predominance in ulcerative colitis in high-incidence areas. Inflammatory bowel disease is less common in persons with occupations involving physical work. Smoking is currently accepted as having a protective effect against ulcerative colitis. However, there seems to be a seasonality in the onset of symptoms in ulcerative colitis, but it is not known whether there is any association between psychiatric factors and ulcerative colitis. There is familial aggregation of inflammatory bowel disease as well as other diseases of unknown origin, such as celiac disease and psoriasis.
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