Incentive relativity is the study of the disappointment and irritation shown by animals and humans when they fail to obtain an expected reward. This book provides a full account of the subject, focusing on animals' responses to the relative value of rewards. These relativity effects cause stress in animals but they may also inspire adaptation beneficial to survival. This text shows how animal research may lead to an understanding of individual differences in discernment and susceptibility to disappointment, and to an understanding of both the advantages and disadvantages of dissatisfaction.