Previous research shows that humans display complexity aversion. In this article, we test for the presence of complexity aversion and its determinants in the context of voluntary climate action, where individual choices interact with the complex regulatory framework of the EU Emission Trading System (ETS). A reform of the ETS in 2018 makes its rules counter-intuitive for its participants, particularly as regards the effectiveness-ranking of alternative public good contributions. Exploiting this feature, we test how individuals act under either no, minimal, or comprehensive information. We find evidence of a small “complexity effect” (less contributions) when no information is provided, but a clear and consistent “learning effect” across all information treatments, regardless of whether they are simple but counter-intuitive or intuitive but complex. Moreover, the learning effect is more pronounced for subgroups whose prior beliefs and moral values run most counter to the actual ranking, i. e. there is no evidence of motivated beliefs. Our findings highlight the importance of communicating the choice-relevant implications of complex contexts to decision makers and show that simple factual statements and detailed explanations are equally effective.