Paying it forward: The development and underlying mechanisms of upstream reciprocity.

Two studies investigated the development and motivations underlying children's upstream reciprocity. In Study 1, 3- and 4-year-olds (n = 40 per age group) received or did not receive help while playing a game. Subsequently, children could share stickers with a new child. The 4-year-olds, but not the 3-year-olds, showed evidence of upstream reciprocity: Those who had received help were more generous toward the new child. Study 2 (N = 46) replicated the results with 4-year-olds and found evidence for a gratitude-like motivation underlying the upstream reciprocity: Children who received help evaluated the benefactor more positively, and positive evaluations of the benefactor correlated with children's upstream reciprocity. Thus, upstream reciprocity emerges by 4 years of age and may already be motivated by a gratitude-like mechanism.