No trust on the left side: Hemifacial asymmetries for trustworthiness and emotional expressions

• The composite face technique was used to investigate hemifacial asymmetries.
• The left side of cheater’s smiling face looked more trustworthy than the right side.
• Cheaters use their left hemiface effectively to conceal their anti-social attitude.

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.