**Stimulus Photography** Photographs were collected using the procedure outlined in [osf.io/bvtnj][1]. **Stimulus matching procedure** From a set of approximately 1500 images of individuals of varying ages, sex and relatedness, we algorithmically chose the maximum number of sibling pairs where both siblings were under the age of 18, and an age-matched (within 1 year), ethnicity-matched, and sex-matched foil image was available from family units that were not represented elsewhere in the image set. Specifically, the two individuals in each sibling pair are related to each other, but not to any other individual in the set, while all individuals in unrelated pairs are related to no individuals in the set. All individuals in the set gave verbal assent and their parent or guardian gave written consent for their images to be used in scientific research. Both smiling and neutral pictures of the children pairs were used in this study. **Stimulus stats** See [childsiblings.csv][2] for details on the sex, age and matching of all stimuli. [1]: http://osf.io/bvtnj [2]: https://osf.io/fs9jp/