Motivation Crowding Theory suggests that external intervention via monetary incentives punishments may undermine, and under different identifiable conditions strengthen, intrinsic motivation. As of today, the theoretical possibility of motivation crowding has been the main subject of discussion among economists. This study demonstrates that the effect is also of empirical relevance. There exist a large number of studies, offering empirical evidence in support of the existence of crowding-out crowding-in exists. The study is based on circumstantial evidence, laboratory studies by both psychologists and economists, as well as field research by econometric studies.