An adaptive response to uncertainty generates positive and negative contrast effects.

Successive contrast effects, in which behavior is dependent on whether conditions are currently better or worse than they were before, are a striking illustration of the fact that animals evaluate the world in relative terms. Existing explanations for these effects are based on descriptive models of psychological and physiological processes, but little attention has been paid to the factors promoting their evolution. Using a simple and general optimality model, we show that contrast effects can result from an adaptive response to uncertainty in a changing, unpredictable world. A wide range of patterns of environmental change will select for sensitivity to past conditions, generating positive and negative contrast effects. Our analysis reveals the importance of incorporating uncertainty and environmental stochasticity into models of adaptive behavior.