<p>This paper reconsiders the role of public prosecutors linking prosecutorial decision-making to incentive structures, institutional constraints, and behavioral biases. Drawing on the classical distinction between false positives and false negatives, we introduce an expected cost function that measures the social costs associated with prosecutorial errors under uncertainty. The model incorporates key variables such as information quality, conviction bias, and prosecutorial care, showing how deviations from the normative ideal—convicting the guilty while protecting the innocent—arise endogenously from misaligned incentives and epistemic limitations. Special attention is given to the principle of the presumption of innocence, reframed as a rational threshold for prosecutorial action. The analysis integrates formal theory with institutional and behavioral insights, and concludes with policy recommendations to improve prosecutorial accountability, reduce systemic error, and reinforce procedural fairness. The framework is applicable across different legal systems and offers a normative basis for institutional reform.</p>