The central proposition of disappointment theory is that an individual forms expectations about uncertain prospects, and that if the actual consequence turns out to be worse than (or better than) that expectation, the individual experiences a sensation of disappointment (or elation) generating a decrement (or increment) of utility which modifies the basic utility derived from the consequence. By incorporating a simple disappointment-elation function into a model of individual choice, many observed violations of conventional expected utility axioms—including violations of Savage's sure-thing principle and the “isolation effect”—can be predicted and defended as rational and dynamically consistent behaviour.