Meeting climate targets requires rapid, widespread reductions in per-capita emissions. Much of that shift requires structural progress, but personal, social, and cultural factors will also play a critical catalytic role in transitions to more climate-friendly behaviors. Existing frameworks often overlook this multilevel context, limiting their ability to predict the outcomes of behavior intervention in real-world settings. Addressing this gap, we introduce a complex-systems framework that captures interactions between five key drivers of behavior change: structural incentives, cultural biases, social influence, history dependence, and attitude-behavior linkage. Using an agent-based model, we systematically explore how the interaction of these drivers shapes population-level behavior dynamics, identifying contexts where different behavior-change interventions may be (in)effective or (in)feasible. This framework maps context-dependence as a design space, yielding guidance for prioritizing and sequencing policy levers. It enables researchers and practitioners to probe large-scale, long-term dynamics that are inaccessible through conventional behavioral-science methods. Teaser Integrating structural, social, and psychological determinants of behavior reveals barriers and opportunities for widespread behavior change.