Human facial expressions as adaptations: Evolutionary questions in facial expression research

Abstract

The importance of the face in social interaction and social intelligence is widely recognized in anthropology. Yet the adaptive functions of human facial expression remain largely unknown. An evolutionary model of human facial expression as behavioral adaptation can be constructed, given the current knowledge of the phenotypic variation, ecological contexts, and fitness consequences of facial behavior. Studies of facial expression are available, but results are not typically framed in an evolutionary perspective. This review identifies the relevant physical phenomena of facial expression and integrates the study of this behavior with the anthropological study of communication and sociality in general. Anthropological issues with relevance to the evolutionary study of facial expression include: facial expressions as coordinated, stereotyped behavioral phenotypes, the unique contexts and functions of different facial expressions, the relationship of facial expression to speech, the value of facial expressions as signals, and the relationship of facial expression to social intelligence in humans and in nonhuman primates. Human smiling is used as an example of adaptation, and testable hypotheses concerning the human smile, as well as other expressions, are proposed.

Fig. 1

Basic facial expression phenotypes. 1, disgust; 2, fear; 3, joy; 4, surprise; 5, sadness; 6, anger. Posed images from Kanade et al. (2000).

Fig. 2

Eyebrow flash of greeting (Eibl-Eibesfeldt, 1989).

Fig. 3

Muscles of facial expression. 1, frontalis; 2, orbicularis oculi; 3, zygomaticus major; 4, risorius; 5, platysma; 6, depressor anguli oris. Original drawing from Huber (1931). (Reprinted by permission of The Johns Hopkins University Press.)

Fig. 4

Non-Duchenne and Duchenne smiles. a: Non-Duchenne smile. b: Duchenne smile. Images from Kanade et al. (2000).

Fig. 5

Automated method for quantifying facial movement. a: Posed smile with outlined features tracked automatically (Tian et al., 2001). b: Movement (radius length relative to mouth center) of right and left lip corners calculated from positional change in tracked features over first 400 msec of expression (Schmidt and Cohn, 2001).

Fig. 6

Homologous displays in human and nonhuman primates. a: Rhesus macaque submissive display. Photograph by Frans DeWaal, 1989.(Silent bared teeth display.) b: Human smile. From Kanade et al.,2000. (Silent bared teeth display.) c: Bonobo play face. Photograph by Frans DeWaal, 1988. (Relaxed open mouth display.) d: Human play face, from Forbes et al., 2000. (Relaxed open mouth display.)