Prospect Theory explained: how people actually make decisions under uncertainty
Prospect Theory is the behavioural model of decision-making under uncertainty developed by Daniel Kahneman and Amos Tversky—published as Prospect Theory: An Analysis of Decision under Risk in Econometrica in 1979. It describes how people actually evaluate outcomes—relative to a reference point, and with asymmetric sensitivity to gains and losses—rather than how the standard economic model of rational choice assumes they do.
What Prospect Theory is
Kahneman and Tversky challenged the expected utility theory that had been the basis of economic models of rational decision-making since the 18th century. Expected utility theory predicts that people evaluate outcomes based on their final wealth level and weight probabilities linearly. Prospect Theory replaces both assumptions with descriptions of how people actually behave, as documented through systematic experimental evidence.
The theory has two core components. The first is a value function: defined relative to a reference point rather than absolute wealth, concave in the domain of gains (producing diminishing sensitivity to further gains), and steeper and convex in the domain of losses (producing heightened sensitivity to losses, especially small ones relative to the reference point). The second is a probability weighting function: people systematically overweight small probabilities and underweight moderate-to-high probabilities.
Tversky and Kahneman (1992), Advances in Prospect Theory: Cumulative Representation of Uncertainty, Journal of Risk and Uncertainty, extended the original model into Cumulative Prospect Theory, addressing some technical limitations while preserving the essential structure. Prospect Theory is the dominant framework in behavioural finance and the foundation from which loss aversion, the disposition effect, and myopic loss aversion are all derived.
How it manifests in investing
The value function directly predicts the disposition effect. Because the function is steeper in the loss domain than the gain domain near the reference point, realising a loss feels disproportionately painful relative to the financial magnitude. Selling a loser makes the loss real; holding it keeps it as a paper loss on the more favourable part of the curve. The investor who holds a deteriorating position is, in the language of the theory, remaining on the part of the value function where further declines hurt most—but the emotional logic of avoiding realisation dominates.
The probability weighting function explains why investors are drawn to lottery-like instruments—high-risk, low-probability, high-payoff positions—even when their expected return is negative. The small probability of a large gain is overweighted relative to its objective likelihood. This produces systematic demand for assets with positively skewed return distributions, and helps explain why speculative assets can sustain prices above what fundamentals support.
Framing effects—a direct prediction of Prospect Theory—are also present in portfolio decisions. The same outcome, framed as a gain relative to one reference point or a loss relative to another, produces different decisions. An investor whose portfolio has fallen 30% and partially recovered to −15% may feel relief at the recovery, even though they remain well below their original entry. The reference point has shifted during the drawdown.
The evidence
The predictions of Prospect Theory have been extensively replicated. The disposition effect—the most direct investment consequence of the value function—has been documented in retail investors (Odean, 1998), professional fund managers (Grinblatt and Keloharju, 2001), and across multiple asset classes and countries. The consistent finding is that investors hold losers too long and sell winners too quickly, and that this pattern produces systematically poor outcomes relative to the alternative. The overweighting of small probabilities has been documented in futures and options markets, lottery preferences, and insurance purchasing behaviour.
What helps
Prospect Theory predicts that investment decisions will be systematically distorted by reference points, asymmetric value weighting, and probability distortions—and that these distortions are universal and largely resistant to awareness alone. The structural mitigation is to remove reference points from the decision process. A systematic strategy that buys and sells based on rules derived from economic signal—momentum, volatility, correlation—does not ask the investor to evaluate whether they are above or below their entry price, or how likely a particular scenario is. The value function and the probability weighting function have no inputs to distort.
Prospect Theory in pfolio
The distortions that Prospect Theory describes—reference-point dependence, asymmetric value weighting, and probability overweighting—operate primarily through discretionary decisions investors make in response to price movements. pfolio's systematic portfolios eliminate those discretionary decisions. Buy and sell signals are generated by rules applied to momentum and volatility data; the investor's position on the Kahneman-Tversky value function—whether they are in a paper gain or paper loss on any individual position—plays no role in the rebalancing calculation. The methodology is fully documented at how we build portfolios.
Related articles
Disclaimer
Get started now
