a longer time than the members of the second group. "Over-performers" continued playing longer than "under-performers" in the first group, but "under-performers" continued playing longer than "over-performers" in the second group. This study showed that, when rewards do not reflect competence, higher rewards lead to less intrinsic motivation. But when rewards do reflect competence, higher rewards lead to greater intrinsic motivation. Richard Titmuss suggested that paying for blood donations might reduce the supply of blood donors. To test this, a field experiment with three treatments was conducted. In the first treatment, the donors did not receive compensation. In the second treatment, the donors received a small payment. In the third treatment, donors were given a choice between the payment and an equivalent-valued contribution to charity. None of the three treatments affected the number of male donors, but the second treatment almost halved the number of female donors. However, allowing the contribution to charity fully eliminated this effect. Theories According to self-perception theory, a person infers causes about his or her own behavior based on external constraints. The presence of a strong constraint (such as a reward) would lead a person to conclude that he or she is performing the behavior solely for the reward, which shifts the person's motivation from intrinsic to extrinsic. Laboratory studies in the 1970s indicated that individuals under conditions with extrinsic rewards showed diminished intrinsic motivation. Deci and his colleagues (e.g., Deci and Ryan 1985) developed the cognitive evaluation theory to explain the results. As a sub-theory of self-determination theory, cognitive evaluation theory explains that both control and competence underlie intrinsic motivation and how extrinsic rewards affect intrinsic motivation is dependent on the individual's interpretation. Intrinsic motivation increases if individuals interpret rewards as pertaining positive information about their own competence and self-control over results, whereas if they interpret the results as indicative of external control, this decreases their feelings of self-control and competence, which in turn decreases intrinsic motivation. Cognitive evaluation theory also suggests social context as another implication on intrinsic motivation. Social cues can exert either positive or negative effects on intrinsic motivation depending on the messages that the context conveys regarding a person's autonomy and competence. Verbal rewards such as positive feedback and praise are predicted to be experienced as controlling hence decreasing intrinsic motivation. However, verbal rewards that are informational and not experienced as controlling are predicted to exert positive effects. Self-determination theory