Several experiments have shown that, when predicting the behaviour of stocks and flows, many subjects rely on the erroneous correlation heuristic. They seem to assume that the output of a system should look the same as the input. Based on similar experiments with kinematics graphs we hypothesize that spatial ability explains variance on tasks involving accumulation. We propose that spatial ability might also generate other important differences between people, such as their ability to infer behaviour from diagrams. We tested participants on two dimensions of spatial ability: visualization and spatial orientation. In an experiment we found that the visualization dimension has a positive effect on performance in various systems thinking inventory tasks and a negative effect on the likelihood that the participant selects a response typical for correlation heuristic reasoning. The positive relation to performance was also present for tasks in which stock behaviour had to be inferred from text and diagrams. Furthermore, we found that people are not persistent in their use of the correlation heuristic between different types of tasks. Males and females did not differ in their spatial ability, but, males did perform better on almost all stock and flow tasks.