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

•In two studies, we examined what motivates upstream reciprocity and when it develops.•Study 1 found that 4-year-olds, but not 3-year-olds, engaged in upstream reciprocity.•Study 2 showed that upstream reciprocity is associated with a key component of gratitude.•These studies suggest that early upstream reciprocity is motivated by a gratitude-like mechanism.