I'll Be There

As promises are non-binding, they can be either fulfilled or broken. Previous lab and field research mostly examines the factors that explain fulfillment, as promising itself is difficult to randomly assign. We design a novel natural field experiment where subtle changes of words in an invitation email shifts the level of promise making among our subjects to test the causal effect of promises. Promises are positively related to experimental attendance, and using the invitation treatment as an instrumental variable, we recover a positive but imprecisely measured causal impact of promise making on behavior.