Smiling and laughter are affective displays. This means they are transitory and most often triggered by some kind of stimulus related to interpersonal communication. In this way acute physiological changes induced by smiling and laughter differ from more enduring bodily effects of sense of humor. A distinction is made between assumptions about physiological processes based on direct observations of expressive behavior and measurements of covert musculoskeletal, respiratory, autonomic, and immunologic, as well as brain, processes related to mirthful laughter.