The evolution and functions of laughter and humor: a synthetic approach.
A number of recent hypotheses have attempted to explain the ultimate evolutionary origins of laughter and humor. However most of these have lacked breadth in their evolutionary frameworks while neglecting the empirical existence of two distinct types of laughter--Duchenne and non-Duchenne--and the implications of this distinction for the evolution of laughter as a signal. Most of these hypotheses have also been proposed in relative isolation of each other and remain disjointed from the relevant empirical literature. Here we attempt to remedy these shortcomings through a synthesis of previous laughter and humor research followed by (i) a reevaluation of this research in light of theory and data from several relevant disciplines, and (ii) the proposal of a synthetic evolutionary framework that takes into account phylogeny and history as well as proximate mechanisms and adaptive significance. We consider laughter to have been a preadaptation that was gradually elaborated and co-opted through both biological and cultural evolution. We hypothesize that Duchenne laughter became fully ritualized in early hominids between 4 and 2 mya as a medium for playful emotional contagion. This mechanism would have coupled the emotions of small hominid groups and promoted resource-building social play during the fleeting periods of safety and satiation that characterized early bipedal life. We further postulate that a generalized class of nonserious social incongruity would have been a reliable indicator of such safe times and thereby came to be a potent distal elicitor of laughter and playful emotion. This class of stimuli had its origins in primate social play and was the foundation for formal human humor. Within this framework, Duchenne laughter and protohumor were well established in the hominid biobehavioral repertoire when more cognitively sophisticated traits evolved in the hominid line between 2 mya and the present. The prior existence of laughter and humor allowed them to be co-opted for numerous novel functions, and it is from this process that non-Duchenne laughter and the "dark side" of laughter emerged. This perspective organizes the diversified forms and functions that characterize laughter and humor today and clarifies when and how laughter and humor evolved during the course of human evolution.