This paper reports an experiment involving an ultimatum bargaining game, played in the Slovak Republic. Financial stakes were varied by a factor of 25, and behavior was observed both when players were inexperienced and as they gained experience. Consistent with prior results, changes in stakes had only a small effect on play for inexperienced players. But the present experimental design allows us to observe that rejections were less frequent the higher the stakes, and proposals in the high stakes conditions declined slowly as subjects gained experience. This Slovak experiment is the first to detect a lower frequency of rejection when stakes are higher and this can be explained by the added power due to multiple observations per subject in the experimental design. A model of learning suggests that the lower rejection frequency is the reason that the proposers in the higher stakes conditions of the ultimatum game learn to make lower offers.

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We present evidence from nearly 14,000 American Red Cross blood drives and from a natural field experiment showing that economic incentives have a positive effect on blood donations without increasing the fraction of donors who are ineligible to donate. The effect increases with the incentive's economic value. However, a substantial proportion of the increase in donations is explained by donors leaving neighboring drives without incentives to attend drives with incentives; this displacement also increases with the economic value of the incentive. We conclude that extrinsic incentives stimulate pro-social behavior, but unless displacement effects are considered, the effect may be overestimated. (JEL D64, H41, I12)

Field-based evidence suggests that guidelines against economic rewards to motivate blood donors should be reconsidered.