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Prospect Theory: An Analysis of Decision under Risk

Why this mattered

Kahneman and Tversky’s paper mattered because it broke the link between rational choice theory and descriptive psychology. Expected utility theory had been the dominant formal account of decision under risk: people were modeled as evaluating final wealth states, weighting outcomes by probabilities, and choosing consistently across equivalent formulations. Prospect Theory showed, with simple experimental choices, that these assumptions systematically failed. People treated gains and losses relative to a reference point, disliked losses more than equivalent gains, overweighted certainty, and changed preferences when identical problems were framed differently. The result was not just a list of anomalies, but a replacement framework precise enough to compete with the older theory.

The paradigm shift was that departures from rational choice could now be modeled, not merely dismissed as error. Prospect theory made it possible to study risk attitudes as psychologically structured: risk aversion for gains and risk seeking for losses followed from the shape of the value function; insurance and gambling could be understood through probability weighting; framing effects became central evidence rather than peripheral noise. This helped open behavioral economics as a field by giving economists, psychologists, and policy researchers a tractable language for predictable irrationality.

Its influence is visible in later breakthroughs across economics and decision science. The paper helped ground research on loss aversion, the endowment effect, status quo bias, mental accounting, behavioral finance, and policy “nudges.” It also set the stage for cumulative prospect theory, which refined the handling of probabilities and became the more widely used formal version. More broadly, the paper changed what counted as an adequate theory of choice: a model now had to explain not only what an ideal agent should choose, but how real people actually evaluate risk, uncertainty, and change.

Abstract

This paper presents a critique of expected utility theory as a descriptive model of decision making under risk, and develops an alternative model, called prospect theory. Choices among risky prospects exhibit several pervasive effects that are inconsistent with the basic tenets of utility theory. In particular, people underweight outcomes that are merely probable in comparison with outcomes that are obtained with certainty. This tendency, called the certainty effect, contributes to risk aversion in choices involving sure gains and to risk seeking in choices involving sure losses. In addition, people generally discard components that are shared by all prospects under consideration. This tendency, called the isolation effect, leads to inconsistent preferences when the same choice is presented in different forms. An alternative theory of choice is developed, in which value is assigned to gains and losses rather than to final assets and in which probabilities are replaced by decision weights. The value function is normally concave for gains, commonly convex for losses, and is generally steeper for losses than for gains. Decision weights are generally lower than the corresponding probabilities, except in the range of low prob-abilities. Overweighting of low probabilities may contribute to the attractiveness of both insurance and gambling. 1.

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