Facial expressions influence kin recognition accuracy

Kinship informs the allocation of pro-social and sexual behaviour. In addition to the ability to detect kin who are directly related to the observer, humans are also able to detect relatedness among others who are not related to themselves based on facial cues of relatedness. However, it is unclear what exact facial cues inform these kinship judgments. Facial expression might be one candidate, as it has been shown that a computer kin-detection algorithm can match relatives accurately when the stimuli are smiling. The current study investigated whether a smiling facial expression increases the accuracy of judging relatedness compared to a neutral facial expression in human raters. The stimuli were images of 50 sibling pairs and 50 unrelated pairs (aged 3-17 years) matched for age, ethnicity and sex. The stimuli included both neutral and smiling versions of each individual. Raters (N=77) were asked to judge whether the presented pairs were related or not in one of two counterbalanced versions of the study, where the same stimuli were never presented as both smiling and neutral to the same rater, and the expression within the pair was always the same. Binary relatedness judgments were analysed using binomial logistic mixed regression. Contrary to expectations, smiling decreased the accuracy of relatedness judgments compared to a neutral facial expression. When shown with a smiling expression compared to a neutral one, related pairs were judged to be related less often, while unrelated pairs were judged to be related more often. Evidence that the upper face is mostly used for kinship judgments suggests that smiles could distort or distract from other, more reliable cues of kinship. Pre-registration, data and code available at https://osf.io/58ewu/