Fasolt V, Holzleitner IJ, O'Shea KJ, Lee A, Jones BC & DeBruine LM (2018). Birth order does not affect ability to detect kin. Human Behaviour and Evolution Society in Amsterdam, Netherlands. July 2018.

Some evidence shows that birth order affects kinship detection ability. Kaminski et al. (2010) argue that firstborns use facial cues combined with 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 only. We replicated this study with raters looking at child siblings rather than parent-neonate sets. 112 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 related to any other image in the study, and individuals from unrelated pairs were not related to any other image. We used binomial logistic mixed effects modelling to predict kinship judgments from relatedness and birth order (with image pair & 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 do not have an advantage in detecting child sibling pairs.

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