Does generosity spur collateral benefits as well as direct ones? I hypothesized that others in one's social network may profit from the charitable spirit and even be inspired to act in kind. In a longitudinal study, participants were randomly assigned to be Givers, Receivers, or Controls. Givers performed acts of kindness over 4 weeks for customized, randomized groups of Receivers who were unaware of the Givers' assignment. Results showed that both Givers and Receivers mutually benefit in well-being from the Givers' practice of generosity, that receiving generosity is an unequivocally positive experience, and that the rewards of practicing generosity spill over to others in the Givers' social networks. Furthermore, Receivers spontaneously chose to practice their own acts of kindness--demonstrating a pay-it-forward effect--and both Givers and Receivers inspired others in their social networks to act kindly towards others. This study advances scientific understanding of how generosity operates in the real world--how individuals practice it, profit from it, and propagate it to others.