The paper describes a hypothesis as to the etiology and pathogenesis of diverticulosis coli. Colonic diverticulosis develops as a result of excessive straining at defecation due to habitual bowel emptying in a sitting posture, which is typical of Western man. The magnitude of straining during habitual bowel emptying in a sitting posture is at least three-fold more than in a squatting posture and upon urge. The latter defecation posture is typical of latrine pit users in underdeveloped nations. The bowels of Western man are subjected to lifelong excessive pressures which result in protrusions of mucosa through the bowel wall at points of least resistance. This hypothesis is consistent with recent findings of elastosis of the bowel wall muscles, the distribution of diverticula along the colon, as well as with epidemiological data on the emergence of diverticulosis coli as a medical problem and its geographic prevalence.