From the American Psychological Association
"Research by psychologists Richard Ryan, PhD, and Edward Deci, PhD, on Self-Determination Theory indicates that intrinsic motivation (doing something because it is inherently interesting or enjoyable), and thus higher quality learning, flourishes in contexts that satisfy human needs for competence, autonomy, and relatedness. Students experience competence when challenged and given prompt feedback. Students experience autonomy when they feel supported to explore, take initiative and develop and implement solutions for their problems. Students experience relatedness when they perceive others listening and responding to them. When these three needs are met, students are more intrinsically motivated and actively engaged in their learning.
"Numerous studies have found that students who are more involved in setting educational goals are more likely to reach their goals. When students perceive that the primary focus of learning is to obtain external rewards, such as a grade on an exam, they often perform more poorly, think of themselves as less competent, and report greater anxiety than when they believe that exams are simply a way for them to monitor their own learning. Some studies have found that the use of external rewards actually decreased motivation for a task for which the student initially was motivated. In a 1999 examination of 128 studies that investigated the effects of external rewards on intrinsic motivations, Drs. Deci and Ryan, along with psychologist Richard Koestner, PhD, concluded that such rewards tend to have a substantially negative effect on intrinsic motivation by undermining people's taking responsibility for motivating or regulating themselves."