"People behave pro-socially in a wide variety of situations that standard economic theory is unable to explain. Social comparison is one explanation for such pro-social behavior: people contribute if others contribute or cooperate as well. This paper tests social comparison in a field experiment at the University of Zurich. Each semester every single student has to decide whether he or shenwants to contribute to two Social Funds. We provided 2500 randomly selected students with information about the average behavior of the student population. Some received the information that a high percentage of the student population contributed, while others received the information that a relatively low percentage contributed. The results show that people behave pro-socially, conditional on others. The more others cooperate, the more one is inclined to do so as well. The type of person is important. We are able to fix the ‘types’ by looking at revealed past behavior. Some persons seem to care more about the pro-social behavior of others, while other ‘types’ are not affected by the average behavior of the reference group."