Fasolt V, Holzleitner IJ, Lee A, O'Shea KJ, Jones BC & DeBruine LM (2017). Facial expression & 3rd Party Kin Recognition. European Human Behaviour and Evolution Association in Paris, France. April 2017.

Kinship informs the allocation of pro-social and sexual behaviour. While cognitive cues, such as co-habitation, play an important role in assessing relatedness, phenotypic cues might also inform relatedness judgments. Research consistently shows that third parties can identify relatedness of individuals from their faces alone at levels somewhat above chance, but these studies differ in more nuanced findings. Most studies used stimuli with a neutral facial expression, however facial expression might be used as a cue of kinship. It has been shown in a computer verification task that the accuracy of matching relatives is high when the stimuli are smiling. Here we investigated whether a smiling facial expression increases the accuracy of judging relatedness compared to a neutral facial expression. Our stimuli (aged 3-17 years) were images of 50 sibling pairs and 50 unrelated pairs matched for age, ethnicity and sex. The stimuli included both neutral and smiling versions of each individual. 81 participants (mean age= 26 years, SD= 12 years) were randomly allocated to one of two counterbalance versions of the study, where the same stimuli were never presented as both smiling and neutral to the same participant. Participants were asked to judge whether the pairs of either smiling or neutral faces were related or not. Contrary to expectations, smiling decreased the accuracy of judging relatedness compared to a neutral facial expression. Related pairs were judged less often to be related, and unrelated pairs were judged more often to be related when smiling compared to neutral. The computer verification task which has highly accurate when judging kinship from facial expression used dynamic smiles rather than static smiles, this difference might have resulted in the varying findings. Moreover, recent research has found that the upper face is mostly used for kinship judgments, hence smiles could distort or distract from other, more reliable cues of kinship.

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