People are significantly more likely to act generously towards a stranger after they observe or receive an act of kindness themselves, according to a study by NSF-funded researchers at Cornell University. However, the researchers also found that those who witnessed many acts of kindness were less likely to be generous to others, a consequence known as the "bystander effect."

Social scientists have long sought to understand the factors that make people more or less likely to engage in generous behavior. This study shows that benefitting from the kindness of strangers can increase and sustain the willingness to "pay it forward." These findings can improve the strategies of governments and philanthropic organizations for promoting helping behavior and cooperation in groups and organizations.

In recent years, social science experiments have demonstrated that single acts of kindness can have ripple effects of promoting more widespread generous behavior. For their research, doctoral student Milena Tsvetkova and sociologist Michael Macy of Cornell University compared the effects that witnessing or benefiting from generous acts had on study participants' generosity toward strangers. They recruited participants for an online experiment that presented them with an opportunity to keep an extra payment for themselves or to donate it anonymously to another participant.

The researchers found that participants who received a donation or witnessed a small number of previous donations between other participants were more likely to act generously and donate their extra payment to others. Participants who witnessed many acts of generosity were actually less likely to help, perhaps because their generosity appeared to be no longer needed. This "bystander effect" was less pronounced among those who had also benefited from a donation, suggesting that receiving acts of kindness may be more important in sustaining chains of generous behavior.