Increasing interconnectedness of people and goods enhances the complexity of geographical problems. For students to understand geography, systems thinking--and in this context, the use of causal diagrams--is a promising approach. A quasi-experimental design is used in which the systems thinking ability of students working with causal diagrams is compared to a control group where students did not work with causal diagrams. Pre- and posttests were taken by 448 students in the experimental group and 168 students in the control group. The results indicate that students in the experimental group outperform students in the control group.