An explanation for motivation crowding-out phenomena is developed in a social preferences framework. Besides selfish and fair or altruistic types a third type of agents is introduced: These quot;conformistsquot; have social preferences if they believe that sufficiently many of the others do too....

By enriching a principal-agent model it is shown that the introduction of monetary incentives may reduce an agent's motivation. In a first step, we allow for the possibility that some agents stick to unverifiable agreements. The larger the fraction of reliable agents, the lower powered will then...