Stock-flow failure is a reasoning error in dynamic systems that has great societal relevance: people misjudge a level of accumulation considering the information on flows that increase or decrease over time. Many interventions, including the use of analogies and graphical manipulations, to counteract this failure and help people integrate the flow information have been tested with little or no success. We suggest that this error relates to the valence of a problem: the framing of the inflow or outflow direction as (...) “good” or “bad” is associated with the direction of its accumulation over time. To explore the effects of valence on accumulation judgments, we employ a scenario of a common health problem: blood glucose control through sugar consumption and insulin flows. We also investigate improvements of performance in a second scenario that result after viewing a video of a dynamic system demonstrating the effects of the correct accumulation trend. We discuss the implications of our findings for the blood glucose example and other global problems. (shrink)