This effect expresses the idea that intrinsic incentives (that is intrinsic motivation) can be crowded out by extrinsic ones, such as monetary rewards. Fehr and Shmidt (1999), Falk and Fischbacher (2000) and Fehr and Gãchter (2002) have emphasized that reciprocity and žother-regarding Ÿ preferences or inequity aversion were possible explanations for those anomalies. According to Kunz and Pfaff (2002), "a common characteristics of all these models is the attempt to explain voluntary cooperation (or dysfunctional effects of explicit incentives respectively) within the very axiomatic of economic theory, e.g. without referring to the concept of intrinsic motivation."" (p. 278). This assertion is half right. First, because even if Bénabou and Tirole (2003) use a principal agent model with full rationality of the two parties, they base their reasoning on the over justification effects that permit the possibility of a detrimental effect of incentives on motivation. Second, because Fehr and .
Gãchter (2002), even if they stress the differences between their own experiments and those made in social psychology in terms of interpretation of the crowding out effect, writes that "as our experiments suggest, explicit incentives may interact in important ways with inequity aversion, bounded rationality and reciprocity motives" (p. 3, our emphasis). .
At any rate, the idea that there could be some possible negative relationships between intrinsic and extrinsic motivation is a highly challenged assumption. Bandura (1986), Eisenberger and Cameron (1996) in psychology, Kunz and Pfaff (2002) or Fehr and Falk (2002) in economics show that the crowding out effect assumption is not sufficiently founded and needs to be avoided as such. .
It seems however that this debate can be settled by resorting to the Self Determination Theory (SDT). This approach indeed permits not only to strengthen the crowding out effect assumption but also to make clear under which conditions, and why a crowding out Ÿ or a crowding in Ÿ effect occurs.
