Add to ORKGThe return from investments in public goods is almost always uncertain, in contrast to the most common setup in the existing empirical literature. We study the impact of natural uncertainty on cooperation in a social dilemma by conducting a public goods experiment in the laboratory in which the marginal return to contributions is either deterministic, risky (known probabilities) or ambiguous (unknown probabilities). Our design allows us to make inferences on differences in cooperative attitudes, beliefs, and one-shot as well as repeated contributions to the public good under the three regimes. Interestingly, we do not find that natural uncertainty has a significant impact on the inclination to cooperate, neither on the beliefs of others nor...