Fasolt V, Holzleitner IJ, Lee A, O'Shea KJ & DeBruine LM (2019). Birth order does not affect ability to detect kin. European Evolution and Human Behaviour Association in Toulouse, France. April 2019.

Some evidence shows that birth order affects kinship detection ability. Kaminski et al. (2010) argue that firstborns use contextual cues (e.g. perinatal association) to assess kinship in their own family, leading to a disadvantage in assessing kinship from facial cues only in strangers. In contrast, laterborns do not have the contextual cue of perinatal association, hence rely more on facial cues, leading to an advantage in detecting kin from facial cues alone. We extended this study to raters judging child siblings, rather than parent-neonate sets. 100 raters viewed 132 pairs of photographs of children (age 3-17), and indicated whether each pair was related or unrelated. Half of the pairs were sibling pairs and half were unrelated child pairs that were age- and gender- matched to a related pair. No image was shown more than once, related pairs were not known to be related to any other image in the study, and individuals from unrelated pairs were not known to be related to any other image. We used binomial logistic mixed effects modelling to predict kinship judgments from relatedness and birth order (with image pair and rater as random factors). Relatedness was the main factor driving kinship judgments; related child-pairs were more than twice as likely as unrelated pairs to be judged as kin. Kinship judgment accuracy was unaffected by rater birth order. These findings indicate that laterborns did not have an advantage in detecting child sibling pairs. Pre-registration, data, code, and preprint available at osf.io/h43ep.

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