Economics students have been shown to exhibit more selfishness than other students. Because the literature identifies the impact of long-term exposure to economics instruction (e.g., taking a course), it cannot isolate the specific course content responsible; nor can selection, peer effects, or other confounds be properly controlled for. In a laboratory experiment, we use a within- and across-subject design to identify the impact of brief, randomly-assigned economics lessons on behavior in games often used to measure selfishness: the ultimatum game (UG), dictator game (DG), prisoner's dilemma (PD), and public-goods game (PGG). We find that a brief lesson that includes the assumptions of self-interest and strategic considerations moves behavior toward traditional economic rationality in UG, PD, and DG. Despite entering the study with higher levels of selfishness than others, subjects with prior exposure to economics instruction have similar training effects. We show that the lesson reduces efficiency and increases inequity in the UG. The results demonstrate that even brief exposure to commonplace neoclassical economics assumptions measurably moves behavior toward self-interest.