The use of mild laws to affect people’s behavior is pervasive – from environmental regulation to tort law – but little is known about how the law changes human behavior and social outcomes when it uses non-deterrent monetary incentives. In a public good experiment, we study the mechanisms that make mild laws to alter human behavior. We identify two kinds of mechanisms: laws may affect behavior by indicating what one should do (i.e. what is appropriate conduct), but laws can also achieve this goal by imposing low monetary incentives. We find that when the low monetary incentive is used in tandem with an indication of what one should do, then the effect on behavior is positive but transitory. The effect is long lasting when we use low monetary incentives in isolation. This suggests that the indication of what one should do makes salient the conflict between people's normative expectations and what others effectively do. This undermines conditional cooperators' own motivation to contribute to public goods. Finally, we compare the effects of mild laws with how mere messages indicating what is moral behavior affect contributions to the public good. Contrary to the existing experimental evidence, we find that messages fail to improve cooperation. We spotlight the conditions under which this is the case.