Publisher Summary Contagious yawning and laughter in humans offer insights into a variety of problems in the neural, behavioral, and social sciences. It does not require conscious effort for an observer to imitate a yawning or laughing person. In the language of classical ethology, these neurological stimulus detectors would be innate releasing mechanisms (IRMs) evolved to detect the releasing stimuli of yawns or laughs. Contagious behavior, thus, provides a novel, noninvasive approach to the neural basis of sensory feature detection. Data collected in the naturalistic tradition of ethology offer considerable potential for clinical research and diagnosis. Armed with baseline data concerning yawning and laughter reviewed in this chapter, one can search for conditions that vary from the normal. The chapter notes the diagnostic significance of yawning for pathologies ranging from brain tumors to hemorrhage. The study of laughter in special and clinical populations offers similar promise. This diverse sample of proposed research converges on the common theme of social communication, its neural substrates, development, evolution, pathology, and variability. The investigation of one aspect of this program illuminates the others.