@unpublished{publication-5741,
	author={Fehr, Ernst and G{\"a}chter, Simon},
	institution={University of Zurich},
	issn={1424-0459},
	number={No. 34},
	series={Working paper series / Institute for Empirical Research in Economics},
	title={Do Incentive Contracts Undermine Voluntary Cooperation?},
	year=2002,
}