ABSTRACT
This chapter postulates that the evolutionary emancipation of silent bared-teeth display from its originally fearful motivation, and the extension of the relaxed open-mouth display from its original motivation of frolicsome playfulness toward affiliative and even polite smoothing of social tension, may have occurred by virtue of a reduction of power asymmetry and an increased overlap of interests among interactants. It presents the hypothesis explaining other parallel developments in the evolution of the social function of displays, for instance the use of "coo" contact calls in different species of macaques. The chapter examines to what extent the variations in facial displays can be attributed to varying characteristics of social organization; that is, how social ecology affects the choice of displays for particular communicatory functions. It shows how the variation in the social function of smile and laughter is related to variation in the social relationships among primates.
