that roughly 50 percent of people increase their
contribution if others do so as well. To our knowledge,
this paper is the first to go further and to test
conditional cooperation in a field experiment.1
Our field experiment about charitable giving
supports the theory of conditional cooperation:
contributions increase, on average, if people
know that many others contribute. The effect
varies, however, depending on past contribution
behavior. Those who never contributed do not
change their behavior, while people who are indifferent
about contributions react most strongly
to information about others’ behavior. Section I
presents the field experiment and the empirical
strategy to test the hypotheses, Section II shows
the results, and Section III offers concluding
remarks.