Prospect Theory

Developed by Daniel Kahneman and Amos Tversky in their 1979 Econometrica paper "Prospect Theory: An Analysis of Decision under Risk," prospect theory was proposed as a descriptive alternative to expected utility theory, which had dominated economics since von Neumann and Morgenstern (1944). Where expected utility assumes rational agents maximize utility over final wealth states, prospect theory observes that real decision-makers systematically deviate from this norm.

The theory rests on three core findings:

• Reference dependence: Outcomes are evaluated as changes relative to a reference point (often the status quo), not as absolute end-states.
• Loss aversion: Losses loom larger than gains of equivalent magnitude—empirical estimates typically place the loss-aversion coefficient around 2 to 2.5.
• Diminishing sensitivity: The value function is concave for gains (risk-averse) and convex for losses (risk-seeking), producing the characteristic S-shaped curve. People also overweight small probabilities and underweight moderate-to-high probabilities through a nonlinear probability weighting function.

A 1992 follow-up, "Advances in Prospect Theory: Cumulative Representation of Uncertainty," refined the framework to handle any number of outcomes and ensure consistency with stochastic dominance, yielding cumulative prospect theory.

Kahneman received the 2002 Nobel Memorial Prize in Economic Sciences for this work; Tversky had died in 1996 and was ineligible. The theory has shaped fields well beyond economics. In international relations, scholars including Jack Levy, Rose McDermott, and Robert Jervis have used it to explain why states take disproportionate risks to avoid losses—for example, leaders escalating failing wars rather than accepting defeat (a "domain of losses" effect). It also underpins policy "nudges," default-option design, and analyses of consumer behavior, insurance markets, and financial bubbles.

Critics note that reference points are often theoretically underspecified and that experimental findings sometimes weaken outside laboratory settings.