Original ArticleIdentifying personality from the static, nonexpressive face in humans and chimpanzees: evidence of a shared system for signaling personality
Introduction
Personality traits describe the stable, context-general behavioral biases of an individual organism. Factor-analytical approaches have identified a small number of human personality traits, with three-factor (Eysenck & Eysenck, 1985) and five-factor (Goldberg, 1993) models being the most used. Human personality as defined by these models has not only cultural but also biological bases. Behavioral genetics studies estimate heritability coefficients for individual traits to be 0.40–0.60 (Bouchard & Loehlin, 2001), and human models show cross-cultural and even cross-species generalization of personality factors (Gosling & John, 1999). In particular, humans and chimpanzees demonstrate similar, although not identical, factor structures. The most important distinction is that the factor-analytical approach identifies an additional (King & Figueredo, 1997), highly heritable (Weiss, King, & Figueredo, 2000) dominance-related factor present in chimpanzees but not in humans.
Many socially relevant traits can be accurately identified in humans solely from visible cues in the static, nonexpressive face, including personality (Kramer & Ward, 2010, Little & Perrett, 2007), sociosexuality (Boothroyd, Jones, Burt, DeBruine, & Perrett, 2008), trustworthiness (Stirrat & Perrett, 2010), and aggression (Carré, McCormick, & Mondloch, 2009). Interpreting these results within animal signaling theory suggests a close association between the facial morphology and behavior of the signal “sender” and the cognitive processes for understanding the signal in the “receiver” (Maynard-Smith & Harper, 2003). Here, we examined the evolutionary history of this signal system. We reasoned that if the face were part of an evolved signal system, then humans and chimpanzees might share aspects of this system. If so, some facial morphology signals should be expressed and understood between species.
The possibility of a shared signal system is plausible in part due to evidence from comparative studies showing similarities in face processing for the two species, including homologous specialized brain regions (Parr, Hecht, Barks, Preuss, & Votaw, 2009), cross-species identification of relatedness (Alvergne et al., 2009), sensitivity to facial configurations (Parr, Heintz, & Akamagwuna, 2006), and homologies in expression (Parr, Waller, Vick, & Bard, 2007). However, a shared signal system of personality would require a variety of other physical and psychological homologies, including behavioral biases as reflected by aspects of personality structure, facial morphology, and the cognitive means for correctly interpreting and using these signals from the face.
To test the possibility of a shared signal system, we measured the ability of humans to understand signals from the static chimpanzee face related to extraversion. Our focus on extraversion was motivated by previous findings. First, in the human face, the signal for extraversion is strong and apparent in both individual (Penton-Voak, Pound, Little, & Perrett, 2006) and composite (Kramer & Ward, 2010) faces. Second, personality characteristics related to human extraversion, such as individual differences in dominance, sociability, and activity levels, are widespread in nonhuman animals (Gosling & John, 1999), including other primates (e.g., chimpanzees; King & Figueredo, 1997), other mammals (e.g., hyenas; Gosling, 1998), fish (e.g., guppies; Budaev, 1997), and even invertebrates (e.g., octopuses; Mather & Anderson, 1993). Finally, the characteristic of “dominant” is encompassed by the trait of “extraversion” in human taxonomy (Goldberg, 1990) and is a particularly robust measure in chimpanzees, demonstrating the single highest factor weighting of any characteristic in the chimpanzee personality model (King & Figueredo, 1997) and both reliability (Freeman & Gosling, 2010) and external validity (Pederson, King, & Landau, 2005) in predicting individual behavior. We carried out a series of four studies in order to investigate accuracy in identifying characteristics relating to extraversion from static chimpanzee faces.
Section snippets
Study 1: accurate personality identification from the chimpanzee face
The first study determined whether people could accurately perceive characteristics relating to extraversion and other personality traits from chimpanzee facial photographs using a forced-choice methodology.
Study 2: accurate identification of dominance in single chimpanzee faces
This study focused more specifically on characteristics relating to Extraversion and Agreeableness, as motivated by the results of Study 1. We used a ratings task in order to incorporate more stimuli and generalize our findings beyond one type of methodology. Single chimpanzee faces were presented and rated for different characteristics. Instead of measuring discrimination accuracy, we measured the strength of correlation between real and perceived characteristics.
Study 3: within-sex discrimination of dominance
Here we asked whether people could accurately distinguish levels of dominance within a single-sex group. We presented images of male and female chimpanzees in separate blocks and used forced-choice discrimination to assess accuracy of dominance perception.
Study 4: comparing accuracy from human and chimpanzee faces
Using forced-choice discrimination, we explored accuracy on chimpanzee dominance and human extraversion in order to see whether performances with these two types of stimuli were related.
General discussion
The results of these four studies demonstrate that humans can accurately perceive characteristics relating to extraversion in chimpanzee faces on the basis of static, nonexpressive cues. In particular, people can use cues in human and chimpanzee faces to identify individuals who are biased toward social activity and dominance-related behaviors.
The ability to detect the characteristic of dominance was an especially robust finding, and it is worth being clear about the nature of this
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