Laws consist of two components: the 'obligations' they express and the 'incentives' designed to enforce them.In this paper we run a public good experiment to test whether or not obligations have any independent effect on cooperation in social dilemmas.The results show that, for given marginal incentives, different levels of minimum contribution required by obligation determine significantly different levels of average contributions.Moreover, unexpected changes in the minimum contribution set up by obligation have asymmetric dynamic effects on the levels of cooperation: a reduction does not alter the descending trend of cooperation, whereas an increase induces a temporary re-start in the average level of cooperation.Nonetheless, obligations per se cannot sustain cooperation over time.