Giving once, giving twice : a two-period field experiment on intertemporal crowding in charitable giving

We study intertemporal crowding between two fundraising campaigns for the same charitable organization by manipulating donors' beliefs about the likelihood of future campaigns in two subsequent field experiments. Theory predicts that the effect of such belief manipulations depends on whether multiple donations are perceived as substitutes or complements. In line with intuition, the data from our experiment in the first year suggests that multiple donations are substitutes and, consequently, that rendering future campaigns more likely causes crowding out. In the second year, however, we find that our belief manipulations no longer have an effect. When receiving a second fundraising call, donors learn that also future repetitions may occur. In contrast with theoretical predictions for substitutes, we find that year-2 donations are increasing in year-1 donations, that is, higher do-nations in an earlier campaign crowd in future donations.