Abstract This manuscript examines how growing up with a sibling relates to prosociality and how knowledge of a partner’s sibling background may serve as a behavioral cue. In a series of experimental games, we found that individuals with siblings were significantly more likely to cooperate in stag hunt and contribute more in public goods and dictator games than only children (OC) on average. In two treatments where a sibling status cue is exogenously revealed, only-child pairs exhibited reduced prosociality. OC exhibit different empirical expectations of behavior compared to those with siblings, while generally sharing the same normative beliefs. Language AI analysis of subjects’ written perspectives on the games corroborates these patterns. We conclude that OC exhibit more context-dependent prosociality, with behavior more closely aligned with empirical expectations than normative beliefs, a pattern not observed in those with siblings.
Babin et al. (2026) studied this question.