Research shows widespread misunderstanding of stocks and flows, even among highly educated adults. People fail to grasp that any stock rises (falls) when the inflow exceeds (is less than) the outflow. Rather, people often use the correlation heuristic, concluding that a system's output is positively correlated with its inputs. Although many argue that system dynamics training will help, evidence is scant. This paper reports an experiment with MIT graduate students to assess the impact of an introductory system dynamics course on intuitive understanding of accumulation. Results show large, statistically significant improvements in overall performance and a reduction in the prevalence of the correlation heuristic. Modest exposure to stocks and flows improves understanding of accumulation, at least among these highly educated adults. However, a minority still show evidence of correlational reasoning. The discussion considers additional experiments to deepen our knowledge of the training required to develop people's intuitive understanding of accumulation. Copyright © 2010 John Wiley & Sons, Ltd.