اعتماد در سمت چپ: عدم تقارن صورت همی برای امانت و ابراز هیجانی

Abstract People can discriminate cheaters from cooperators by their appearance. However, successful cheater detection can be thwarted by a posed smile, which cheaters display with greater emotional intensity than cooperators. The present study investigated the underlying neural and cognitive mechanisms of a posed smile, which cheaters use to conceal their anti-social attitude, in terms of hemifacial asymmetries of emotional expressions. Raters (50 women and 50 men) performed trustworthiness judgments on composite faces of cheaters and cooperators, operationally defined by the number of deceptions in an economic game. The left–left composites of cheaters were judged to be more trustworthy than the right–right composites when the models posed a happy expression. This left-hemiface advantage for the happy expression was not observed for cooperators. In addition, the left-hemiface advantage of cheaters disappeared for the angry expression. These results suggest that cheaters used the left hemiface, which is connected to the emotional side of the brain (i.e., the right hemisphere), more effectively than the right hemiface to conceal their anti-social attitude.

1. Introduction The ability to discriminate potential cooperators from cheaters is fundamental for social cooperation, which is a universal feature of human societies (Cosmides, 1989 and Trivers, 1971). Previous studies have demonstrated that people can successfully detect cooperative signals, or trustworthiness, just by inspecting facial photographs (e.g., Okubo et al., 2012 and Verplaetse et al., 2007), by viewing videoclips of natural conversations (Oda, Yamagata, Yabiku, & Matsumoto-Oda, 2009), or by viewing participants reading a short story aloud (Brown, Palameta, & Moore, 2003). Todorov and his colleagues have found a relationship between perceptions of facial trustworthiness and emotional expressions (Engell et al., 2010, Oosterhof and Todorov, 2008, Oosterhof and Todorov, 2009, Todorov et al., 2008 and Todorov and Duchaine, 2008). Todorov and Duchaine (2008) demonstrated that perceived facial trustworthiness was positively correlated with judgments of happiness. Oosterhof and Todorov (2008) identified trustworthiness and dominance as fundamental dimensions of face evaluation using a principal component analysis of judgments on various facial traits, and then developed computer models for representing how faces varied on these two fundamental dimensions. Based on their computer models, Oosterhof and Todorov (2009) created computer-generated faces and continually changed their facial trustworthiness. They found that lower trustworthiness increased the intensity of perceived anger whereas greater trustworthiness increased the intensity of perceived happiness. Engell et al. (2010) found that perceptual adaptation to an angry face increased the subsequent evaluation of trustworthiness of a face with a neutral expression, while adaptation to a happy face decreased the subsequent evaluation of trustworthiness. These results suggest that facial trustworthiness is evaluated on the basis of subtle facial cues that resemble emotional expressions signaling cooperativeness (Engell et al., 2010, Oosterhof and Todorov, 2009 and Todorov and Duchaine, 2008). The positive correlation between perceptions of facial trustworthiness and emotional expressions has been replicated in other laboratories (Krumhuber, Manstead, Cosker, Rosin, & Marshall, 2007; Ozono et al., 2010). Okubo et al. (2012) recently found that facial photographs of cheaters were rated to be less trustworthy than those of cooperators when the models expressed anger on their faces. However, cheater detection was unsuccessful when the models posed a smile (Okubo et al., 2012). In addition, the photographs of cheaters were rated to have higher emotional intensity than those of cooperators, irrespective of facial expressions. These results suggest that cheaters can cunningly conceal their anti-social attitude from observers by use of a posed smile, which they put on with greater intensity than do cooperators. The purpose of the present study was to investigate neural and cognitive mechanisms that underlie the posed smile of cheaters in terms of hemispheric and hemifacial asymmetries of emotional expressions. Previous studies have demonstrated that emotions are expressed more intensely in the left hemiface than in the right hemiface (e.g., Indersmitten and Gur, 2003, Kowner, 1995, Nicholls et al., 2004, Sackeim et al., 1978 and Zaidel et al., 1995). For example, a left hemiface advantage for emotional expression was found for the rating of emotional faces in video clips (e.g., Borod, Kent, Koff, Martin, & Alpert, 1988) and in 3-D computerized image analysis of emotional faces (Nicholls et al., 2004). These left hemiface advantages can be attributed to the right hemisphere (RH) dominance of emotional expressions (for a review, see Demaree, Everhart, Youngstrom, & Harrison, 2005). The RH dominance of muscle movement in the left hemiface should also contribute to the left-hemiface advantage for emotional expressions (Patten, 1996). The composite face technique is widely used to investigate the left-hemifacial advantage for emotional expressions, which originates in the RH (e.g., Indersmitten and Gur, 2003, Kowner, 1995, Sackeim et al., 1978 and Zaidel et al., 1995). A composite face is composed of a left or right hemiface and its mirrored (left or right) hemiface, resulting in a left–left or right–right composite (see Fig. 1). Previous studies have repeatedly demonstrated that left–left composites are judged to be emotionally more expressive than right–right composites (e.g., Indersmitten and Gur, 2003, Kowner, 1995, Sackeim et al., 1978 and Zaidel et al., 1995). Although such an advantage for left–left composites have been observed both for posed and spontaneous expressions (for a review, Skinner & Mullen, 1991), the advantage of left–left composites was more consistent across emotions for posed emotions than for spontaneous ones (Indersmitten and Gur, 2003 and Mandal et al., 2001). Examples of left–left (left) and right–right (right) composites for happy (top), ... Fig. 1. Examples of left–left (left) and right–right (right) composites for happy (top), neutral (middle) and angry (bottom) expressions. Figure options Nicholls, Clode, Wood, and Wood (1999) found that participants tended to present the left side of their face when asked to portray as much emotion as possible for a family portrait, whereas they tended to present the right side when asked to pose as a scientist and avoid portraying emotion. Since the left hemiface is more emotionally expressive than the right hemiface (e.g., Indersmitten and Gur, 2003, Nicholls et al., 2004 and Sackeim et al., 1978), the posing biases observed by Nicholls et al. (1999) can be explained on the basis of participants’ motivation to express or conceal emotion: People may tend to adopt the most efficient means of using their hemispheric functions. We expect that cheaters also try to utilize their hemispheric functions most effectively in concealing their anti-social attitude. Because emotions are expressed more intensely in the left hemiface than in the right hemiface (e.g., Borod et al., 1988, Indersmitten and Gur, 2003 and Nicholls et al., 2004), we hypothesized that cheaters would use the left-hemiface, which is connected to the emotional side of the brain (i.e., RH), more effectively than the right hemiface in concealing their anti-social attitude by use of a posed smile. To test this hypothesis, we created composite faces using photographs of the faces of cheaters and cooperators with happy, neutral, and angry expressions, and conducted trustworthiness judgments on the composite faces. On the basis of our hypothesis, we predicted that the left–left composites of cheaters with happy expression would be judged to be more trustworthy than their right–right composites. Such a left-face bias would decrease for the composite faces of cooperators. In contrast, because anger is negatively related to facial trustworthiness (e.g., Todorov & Duchaine, 2008), the left-face bias for cheaters would be eliminated or even reversed for composite faces with the angry expression. We also investigated the effect of sex on perception of trustworthiness in this study for the following reasons. First, women are better able than men to judge sexual unfaithfulness from face photographs of opposite-sex strangers (Rhodes, Morley, & Simmons, 2013). Second, women, relative to men, are generally more sensitive to subtle changes in emotional expressions (for a review, see Sasson et al., 2010). Third, men are usually more lateralized than women in terms of cognitive as well as emotional functions (see, Borod et al., 1998 and Kimura, 2000). On the basis of these sex differences, the pattern of results predicted above should be more pronounced in female raters than in male raters

Concluding comments Okubo et al. (2012) demonstrated that successful cheater detection was thwarted by a posed smile, which cheaters display with greater emotional intensity than cooperators. Using the face composite technique, the present study demonstrated that the left–left composites of cheaters with happy expression were judged to be more trustworthy than their right–right composites and such a left-face bias would decrease for the composite faces of cooperators. Considering the relationship between the RH and the left hemiface (for a review, Skinner & Mullen, 1991), these results suggest that the RH plays a crucial role to conceal cheaters’ anti-social attitude by use of a posed smile.