Contributions to public goods can be motivated by intrinsic factors such as warm glow altruism and fairness, as well as extrinsic incentives such as sanctions and payments. However, psychological studies suggest that formal extrinsic incentives may crowd out intrinsic motivations. In an experimental study of individual contributions to a public good we find that suasion crowded in voluntary contributions, while an extrinsic incentive in the form of a regulation led to crowding out. This has implications for the design of public policy where ranges of motivations are present.