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According to the researchers, laughter seems to cause the tissue that forms the inner lining of blood vessels to relax or expand, increasing blood flow. Mental stress causes the opposite, making vessel linings constrict, thus reducing blood flow. This finding confirmed earlier studies, suggesting a link between emotional stress and the narrowing of these linings, called the endothelium.

"The endothelium is the first line in the developing of atherosclerosis, or hardening of the arteries," said Dr. Michael Miller, principal investigator for the study, "so, given the results of our study, it is conceivable that laughing may be important to maintain a healthy endothelium and reduce the risk of cardiovascular disease. At the very least, laughter offsets the impact of mental stress, which is harmful to the endothelium." Miller is director of preventive cardiology at the University of Maryland Medical Center and an associate professor of medicine. The study was presented before the American College of Cardiology's scientific meeting in Orlando, Florida.