Using a survey of a large group of first- and final-year students of different disciplines to study their beliefs in the existence of mutual benefits of market transactions, the authors observe significant differences between economics and business students versus students of other disciplines. These differences increase over time, due partly to economics students increasingly supporting the belief and partly to other students, in particular psychology students, increasingly disagreeing with it. The beliefs of economics students are more homogeneous at the end of their studies. The authors, therefore, report evidence of both a selection effect and an effect of studying different disciplines that goes beyond initial self-selection.