<p>Although many preventive health interventions are highly cost-effective, participation and sustained engagement remain below recommended levels. This paper advances prevention science by offering a behavioral explanation for underinvestment in prevention at the level of individual preventive behavior, specifically, participation and adherence, rather than system-level funding decisions. Drawing on behavioral economics, it integrates two well-supported mechanisms, present bias and mental accounting, to explain failures both in initiating preventive actions and in maintaining them over time. The central argument is that underinvestment reflects a dual-margin behavioral problem. Mental accounting shapes whether individuals perceive preventive services as affordable in terms of monetary, time, effort, and psychological costs, even in settings where financial prices are zero or services are publicly funded. Present bias influences effort and persistence by causing individuals to overweight immediate inconveniences relative to future health benefits. By explicitly distinguishing individual-level decision-making from policy maker behavior, the framework clarifies its scope and avoids conflating distinct decision problems. Distinguishing between initiation and follow-through helps explain why many prevention policies increase uptake without producing sustained behavior change. The paper offers a conceptual framework for interpreting mixed findings in prevention research and develops testable predictions about when it is better to focus on participation versus adherence.</p>