Facial similarity between individuals informs kinship judgments in third-party kin recognition. Indeed, one study found that similarity and kinship judgments encapsulate the same information (Maloney & Dal Martello, 2006). Yet, another study found that this is not the case when comparing adult face pairs of different sex (DeBruine et al., 2009). We replicated these studies to further clarify the role of facial similarity in kin recognition. We recruited 318 raters, who were shown 50 sibling pairs and 50 age-and sex-matched unrelated pairs ranging from 3 to 17 years old. Each rater was randomly assigned to make either kinship judgments ("related" or "unrelated") or similarity judgments (scale from 0 [not very similar] to 10 [very similar]). The threshold model found that performance in both tasks was equally accurate, with participants detecting child siblings in the kinship task above chance and giving significantly higher similarity ratings to siblings in the similarity task. In both tasks, opposite-sex siblings were perceived to be siblings less often than same-sex siblings, and judgments of unrelated face pairs were not affected by the sex of faces. Conversely, the effect of age difference within pairs of faces differed for the two tasks: a greater age difference decreased all kinship judgments, but only decreased similarity judgments of siblings, not unrelated pairs. In line with DeBruine et al. (2009), these findings suggest that similarity and kinship judgments are highly correlated but not strictly synonymous. The OSF Pre-registration Challenge for this project can be found at osf.io/ps9hy and the data at osf.io/sef9k.

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Kinship and facial self-resemblance evoke similar patterns of behaviour, such as increased pro-sociality and decreased sexual motivation. Moreover, it has been suggested that facial resemblance/similarity between individuals informs kinship judgments in third-party kin recognition as one study found that similarity and kinship judgments encapsulate the same information (Maloney & Dal Martello, 2006). Yet, another study found that similarity judgments convey some information that is not conveyed in kinship judgments when comparing adult face pairs of different sex (DeBruine et al., 2009). We replicated these studies to further investigate these convergent results and clarify the role facial similarity plays in third-party kin recognition. We recruited 318 raters, who were shown 50 sibling pairs and 50 age- and sex-matched unrelated pairs ranging from 3 to 17 years old. Each rater was randomly assigned to make either kinship judgments (“related” or “unrelated”) or similarity judgments (scale from 0-“not very similar” to 10-“very similar”). We found that performance in both tasks was equally accurate, with participants detecting child siblings in the kinship task above chance and giving significantly higher similarity ratings to siblings in the similarity task. In both tasks, opposite-sex siblings were less often perceived to be siblings less often than same-sex siblings, while judgments of unrelated face pairs were not affected by the sex of faces. In contrast, the effect of difference in age between pairs of faces differed for the two tasks: in the similarity task, sibling pairs’ similarity decreased with increasing age differences, while no such effect was observed for unrelated pairs. In the kinship task, an increasing age difference decreased relatedness judgments for both related and unrelated pairs. In line with DeBruine et al. (2009), these findings suggest that similarity and kinship judgments are highly correlated but not strictly synonymous. The OSF Pre-registration for this project can be found at osf.io/ps9hy and the data and code at osf.io/sef9k.

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