Smiles are the most contagious of all emotional signals. Also describes how laughter is contagious, as well, and how it demonstrates the open loop limbic system. (Goleman, Daniel, PhD, with Richard Boyatzis, and Annie Mckee. Primal Leadership. p 6-8. Boston: Harvard Business School Press, 2002)
Laughter is contagious. The human brain contains detectors that respond to laughter by triggering other neural circuits in the brain that generate more laughter. (Bartekian, Vatche. Laughter Can Cure What Ails You. Ask Men web site.)
Laughter is contagious; often laughter alone will provoke laughter from others. THis may have formed the basis for the development of "laugh tracks."