During the last decade, the number of institutions for collective action (e.g., self-organized insurance in mutuals) has risen significantly across Europe, and in the Netherlands in particular. Why do some initiatives succeed, while others fail? To what extent does success depend on the individuals involved or the initiative’s social and institutional properties? In this project, we aim to answer these questions by combining individual, social, and institutional factors in a multidisciplinary approach. Abstract experiments and observations of real-life behavior are combined to derive multi-faceted causal evidence on success factors for collective action, specifically aiming to uncover institutional opportunities for optimizing success of mutuals.