Kin recognition based on viewing photographs of children's faces is not affected by facial inversion

Background: Many facial tasks such as identity recognition and judgments of emotion are more difficult when faces are inverted. We report an objective task (judgment of kinship of pairs of children) where task performance does not deteriorate with inversion of stimuli. Methods: 30 pairs of color photographs, each photograph portraying a frontal view of a child's face with background removed. Half of the pairs were siblings, half unrelated. The children's ages spanned 14 years. The distribution of age differences for related and unrelated pairs was matched as were gender differences. A total of 118 adult observers judged each pair as siblings or not. We summarized performance in each condition by signal detection d′ estimates. Conditions: Upright face (UF); face flipped around a horizontal axis (IF); 180° rotated face (RF). Different observers participated in each condition. Results: Performance in the UF condition (d′ = 1.058), IF condition (d′ =1.197), and RF condition (d′ = 1.048) were not significantly different (p > 0.05). Likelihood criterion (β) values of UF (β = 0.967), IF (β = 1.054), RF (β = 1.033) also did not differ significantly (p > 0.05). The β values of UF, IF and RF were not significantly different from 1 (p > 0.05): there was no significant bias in classifying children as related or unrelated. Conclusion: Allocentric kin recognition is not disrupted by inversion of the facial stimuli: the judgment appears to rely on inversion invariant cues. We discuss this result in conjunction with previous work (Dal Martello & Maloney, 2006; DeBruine et al., 2009) concerning localization of kin recognition cues in the face.