Intrinsic motivation has been studied since the early 1970s. Intrinsic motivation is a behavior that is driven by satisfying internal rewards. For example, an athlete may enjoy playing football for the experience, rather than for an award. It is an interest or enjoyment in the task itself, and exists within the individual rather than relying on external pressures or a desire for consideration. Deci (1971) explained that some activities provide their own inherent reward, meaning certain activities are not dependent on external rewards. The phenomenon of intrinsic motivation was first acknowledged within experimental studies of animal behavior. In these studies, it was evident that the organisms would engage in playful and curiosity-driven behaviors in the absence of reward. Intrinsic motivation is a natural motivational tendency and is a critical element in cognitive, social, and physical development. The two necessary elements for intrinsic motivation are self-determination and an increase in perceived competence. In short, the cause of the behavior must be internal, known as internal locus of causality, and the individual who engages in the behavior must perceive that the task increases their competence. According to various research reported by Deci's published findings in 1971, and 1972, tangible rewards could actually undermine the intrinsic motivation of college students. However, these studies didn't just affect college students, Kruglanski, Friedman, and Zeevi (1971) repeated this study and found that symbolic and material rewards can undermine not just high school students, but preschool students as well.