Perceived Differences Between Chimpanzee (Pan troglodytes) and Human (Homo sapiens) Facial Expressions Are Related to Emotional Interpretation Bridget M. Waller University of Portsmouth Kim A. Bard University of Portsmouth Sarah-Jane Vick University of Stirling Marcia C. Smith Pasqualini Avila University Human face perception is a finely tuned, specialized process. When comparing faces between species, therefore, it is essential to consider how people make these observational judgments. Comparing facial expressions may be particularly problematic, given that people tend to consider them categorically as emotional signals, which may affect how accurately specific details are processed. The bared-teeth display (BT), observed in most primates, has been proposed as a homologue of the human smile (J. A. R. A. M. van Hooff, 1972). In this study, judgments of similarity between BT displays of chimpanzees (Pan troglodytes) and human smiles varied in relation to perceived emotional valence. When a chimpanzee BT was interpreted as fearful, observers tended to underestimate the magnitude of the relationship between certain features (the extent of lip corner raise) and human smiles. These judgments may reflect the combined effects of categorical emotional perception, configural face pro- cessing, and perceptual organization in mental imagery and may demonstrate the advantages of using standardized observational methods in comparative facial expression research. Keywords: facial expression, emotion, primates, perception, smiling Observation is a key method in comparative psychology and animal behavior, and yet the tools we use to observe—namely, our senses and related perceptual processing systems—are rarely con- sidered in relation to their application in scientific methods. Spe- cifically, it is crucial that our skills in face perception are taken into account within the field of comparative facial expression research. In the present study, we investigated the processes at work when observers make comparisons between chimpanzee and human facial expressions. The bared-teeth (BT) display (see Figure 1) is a facial expres- sion common to most primates and has been proposed as a homo- logue to the human smile (Andrew, 1963; van Hooff, 1972). Assessments of homology have been based on appearance (An- drew, 1963; Preuschoft & van Hooff, 1995; van Hooff, 1972), similarity of social function (Preuschoft, 1995; van Hooff, 1972; Waller & Dunbar, 2005) and muscular basis (Waller et al., 2006). Some authors, however, feel that although morphologically simi- lar, the emotional valence of the two expressions differs (e.g., Bard, 2000). Redican (1982) termed the BT display “fear gri- mace,” which is now a commonly used term within the field, and also suggested that rather than featuring raised and upturned mouth corners (like the human smile), the BT has more down-turned and laterally pulled mouth corners. He suggested that this configura- tion is more similar to the human fear expression (or grimace) and is likely due to action of the risorius muscle as opposed to the zygomatic major muscle (which is the central movement underly- ing the human smile; Waller et al, 2006). This controversy high- lights two important issues. First, without a system to standardize these scientific descriptions, we are unable to quantify the true degree of physical similarity between these proposed homologues. Second, it seems possible that there is a relationship between how scientists perceive the physical characteristics of other species’ faces and how they interpret them emotionally. Hebb (1946) argued that human observation can be used to infer emotion from animal behavior reliably, yet Foley (1935) found huge variation in nonexpert participants’ emotional categorization of chimpanzee facial expressions. The reason this anthropomor- phic approach may be particularly problematic when viewing faces could be due to the streamlined manner in which faces are pro- cessed. Face recognition has been a subject of inquiry within psychology for decades, and although initially this focused mainly on how faces are recognized in terms of identity (Bruce & Young, Bridget M. Waller and Kim A. Bard, Centre for the Study of Emotion, Department of Psychology, University of Portsmouth, Portsmouth, Hamp- shire, United Kingdom; Sarah-Jane Vick, Department of Psychology, Uni- versity of Stirling, Stirling, Scotland, United Kingdom; Marcia C. Smith Pasqualini, Department of Psychology, Avila University. This research was supported by a Research Interchange grant from The Leverhulme Trust to Kim A. Bard. We would like to thank Samuel Fernandez-Carriba for many helpful comments and for providing images; Lisa Parr and William Hopkins for providing images; Maxine Wesley for providing computer programming expertise; and Jan van Hooff and Alan Costall for numerous constructive comments. Thanks are also extended to The Chester Zoo, North of England Zoological Society, Chester, United Kingdom, for allowing us to collect and use images of their chimpanzees. Correspondence concerning this article should be addressed to Bridget M. Waller, Department of Psychology, King Henry Building, University of Portsmouth, Portsmouth PO1 2DY, United Kingdom. E-mail: bridget.waller@port.ac.uk Journal of Comparative Psychology Copyright 2007 by the American Psychological Association 2007, Vol. 121, No. 4, 398 – 404 0735-7036/07/$12.00 DOI: 10.1037/0735-7036.121.4.398 398