Lay theory interventions instill situation-general ways of thinking, often using short reading and writing exercises, and they have led to lasting changes in behavior and performance in a wide variety of policy domains. Do they work in all contexts? We suggest that lay theory intervention effects depend on psychological affordances, which are defined as cues that allow individuals to view a lay theory as legitimate and adaptive in that context. The present research directly and experimentally tested this hypothesis using the example of a “purpose for learning” lay theory intervention, which taught the lay theory that school is a place to develop skills that allow one to make progress toward self-transcendent aims. A double-blind 2 (student purpose intervention) × 2 (purpose-affording note) field experiment was conducted in a relatively low-performing public middle school in the United States. Students first received a web-based purpose for learning lay theory intervention (or a control activity), and 2 weeks later attended a class in which an assignment was accompanied by a purpose-affording note that was hand-written by a teacher (or a control note). Results showed that the purpose lay theory intervention increased performance on an English class writing assignment, but only when it was accompanied by a purpose-affording note. Exploratory analyses revealed that the effects of the manipulations were apparent among students who were at greater risk for poor performance in the class: nonnative English-speaking students. Thus short, online lay theory interventions may reduce performance gaps, provided that the contexts afford the opportunity for the proffered lay theory to seem legitimate and adaptive.