History of psychology students wrote essays about historical figures and counterfactual events. A linguistic analysis of the essays revealed that counterfactual assignments included more auxiliary verbs and more references to tentativeness and the future. More important, scores on the counterfactual assignments but not the historical figure assignments correlated positively with a critical thinking subtest of the Collegiate Assessment of Academic Proficiency. The linguistic analysis and standardized test results suggest that the counterfactual assignment promotes a different form of thinking than more traditional historical assignments. Counterfactual thinking assignments may be applied to courses in other areas of psychology.