The experiment will be conducted online and participants will be recruited using Prolific. Participants are operating within groups of size three in which they have to solve three tasks (treatments). In each task they have to decide how much to contribute to each of four public goods. In the first task the marginal per capita returns (MPCR) of each of the public goods are known (“certainty”); in the second task the MPCRs are subject to known uncertainty (“risk”); and in the third task the MPCRs are subject to unknown uncertainty (“ambiguity”). Participants will be rewarded according to one of the tasks, but according to all four public goods within this task. The MPCRs are chosen such that cost-effectiveness can be unambiguously determined for all treatments, independent of risk/ambiguity attitude and of individuals’ beliefs about contributions by others. After having solved these tasks, there will be a short questionnaire with some demographic questions (gender and age), an elicitation of their giving-type, their risk and ambiguity attitude, and their attention level and risk literacy. Participants record all their decisions in the computer software (programmed in oTree).