Robert R. Provine

Laughter is a simple, stereotyped, innate, human play vocalization that is ideal for the study of vocal evolution. The basic approach of describing the act of laughter and when we do it has revealed a variety of phenomena of social, linguistic, and neurological significance. Findings include the acoustic structure of laughter, the minimal voluntary control of laughter, the punctuation effect (which describes the placement of laughter in conversation and indicates the dominance of speech over laughter), and the role of laughter in human matching and mating. Especially notable is the use of laughter to discover why humans can speak and other apes cannot. Quadrupeds, including our primate ancestors, have a 1:1 relation between breathing and stride because their thorax must absorb forelimb impacts during running. The direct link between breathing and locomotion limits vocalizations to short, simple utterances, such as the characteristic panting chimpanzee laugh (one sound per inward or outward breath). The evolution of bipedal locomotion freed the respiration system of its support function during running, permitting greater breath control and the selection for human-type laughter (a parsed exhalation), and subsequently the virtuosic, sustained, expiratory vocalization of speech. This is the basis of the bipedal theory of speech evolution.

Laughter is a stereotyped, innate, human play vocalization that provides an ideal simple system for neurobehavioral analyses of the sort usually associated with such animal models as walking, wing‐flapping, and bird song. Laughter research is in its early stages, where the frontiers are near and accessible to simple observational procedures termed “sidewalk neuroscience.” The basic, nontechnical approach of describing the act of laughter and when humans do it has revealed a variety of phenomena of social and neurological significance. Findings include the acoustic structure of laughter, the minimal voluntary control of laughter, contagiousness, the “punctuation effect” that describes the placement of laughter in conversation, the dominance of speech over laughter, the role of breath control in the evolution of speech, the evolutionary trajectory of laughter in primates, and the role of laughter in human matching and mating. If one knows where to look and how to see, advances in neuroscience are accessible to anyone and require minimal resources. J. Comp. Neurol. 524:1532–1539, 2016.

Contagious behaviors such as yawning and itching/scratching have mirror-like properties and clearly defined stimulus and motor parameters; they are also relatively easy to study and should be part of the debate about mirror neurons and the neurological mechanisms of social behavior. The broadly tuned, multimodal stimuli of contagious behavior challenge present accounts of mirror mechanisms that focus on specific, mirrored acts.

Among primates, only humans have the white sclerae that provide the ground necessary to display their own color and that of the overlying conjunctiva. Scleral color, primarily redness, provides cues of socially and biologically significant information about an individual. The present study examines the effect of the asymmetry of binocular scleral redness on perceived sadness, healthiness, and attractiveness by contrasting ratings of images of individuals who had one, both, or neither sclera reddened by digital editing. Building upon previous research, this study further defines the details of the scleral color display and contributes to the more general issue of facial and body symmetry, predictors of phenotypic condition and genotypic quality that are of interest to evolutionary theorists. Individuals with binocular and monocular redness were rated as sadder, less healthy, and less attractive than those with untinted control sclerae, with ratings corresponding to the degree of redness or whiteness. Bilaterally symmetrical (binocular) scleral redness or whiteness provided anchorage points for ratings, with ratings for bilaterally asymmetrical (monocular) redness or whiteness falling between them; there was no unique effect of asymmetry.

The sclera, the eye's tough white outer layer, provides the ground necessary for the display of its own color and that of the overlying membrane, the conjunctiva. This study evaluated the sclera as a cue of emotion by contrasting the ratings of 38 subjects for the level of anger, fear, sadness, disgust, happiness, or surprise of normal (untinted) eye images with copies of those images that were reddened by digital editing. Subjects rated individuals with reddened sclera as having more anger, fear, disgust, and sadness, and less happiness than those with normal, untinted sclera. Surprise was the only emotion unaffected by scleral redness. Humans, but not other primates, have evolved the white sclera necessary to display the blood flow in the overlying conjunctiva that produces the redness associated with certain emotional states.

The sclera, the eye’s tough outer layer, is, among primates, white only in humans, providing the ground necessary for the display of colors that vary in health and disease. The current study evaluates scleral color as a cue of socially significant information about health, attractiveness, and age by contrasting the perception of eyes with normal whites with copies of those eyes whose whites were reddened, yellowed, or further whitened by digital editing. Individuals with red and yellow sclera were rated to be less healthy, less attractive, and older than individuals with untinted control sclera. Individuals with whitened, “super-white” sclera were rated as younger, although not more healthy or attractive, than controls. In humans, clear, white sclera may join such traits as smooth skin and long, lustrous hair as signs of health, beauty, and reproductive fitness. The evolution of a white sclera may have contributed to the emergence of humans as a social species.

Laughing, humor, and tickling, but not tearing and crying, involve the give-and-take that provides value and a basis for exchange in the psychosexual marketplace.

Tearing is not a benign secretory correlate of sadness or other emotional state, but a potent visual cue that adds meaning to human facial expression, the tear effect. Although tearing (lacrimation) provides ocular lubrication and is a response to irritation in many animals, emotional tearing may be unique to humans and does not develop until several months after birth. This study provides the first experimental demonstration that tears are a visual signal of sadness by contrasting the perceived sadness of human facial images with tears against copies of those images that had the tears digitally removed. Tear removal produced faces rated as less sad. Anecdotal findings suggest further that tear-removal often produced faces of uncertain emotional valence, perhaps awe, concern, or puzzlement, not just less sad. Tearing signaled sadness and resolved ambiguity. The evolution and development of emotional tearing in humans provide a novel, potent and neglected channel of affective communication.