When designing and implementing policies, policy makers usually assume linear, proportionate causation between interventions and consequences. Yet frequently unexpected consequences occur that seem unintended and disproportionate. This article argues that interventions are more appropriately understood as loops, not lines. System dynamics shows that causes and consequences interact in circular patterns, creating unexpected outcomes and self-reinforcing mechanisms. Some loops are vicious, causing deterioration of the situation, others are virtuous, propelling self-sustaining improvements that exceed original intentions. The article illustrates the circular approach to causality by applying it to interventions aimed at the improvement of the performance of primary schools in the Netherlands.