Classical economics assumes decision-makers maximise expected utility using probability-weighted outcomes. Real humans systematically violate these assumptions in specific, predictable ways. Prospect theory, developed by Daniel Kahneman and Amos Tversky in 1979 and refined subsequently, provides a mathematical framework for how humans actually make decisions under uncertainty. Understanding prospect theory explains most of what looks like irrational investor behaviour but is actually the predictable output of a specific psychological system.
The classical expected-utility framework
Classical economic theory assumes decision-makers choose among uncertain outcomes by computing expected utility — the probability-weighted sum of the utility values of each possible outcome — and selecting the option with the highest expected utility. This framework has elegant mathematical properties and produces predictions about behaviour that could, in principle, be tested empirically.
The tests have consistently found that human behaviour violates the framework in specific ways. The violations are not random errors that average out; they are systematic patterns that appear across cultures, income levels, and cognitive backgrounds. Prospect theory provides a mathematical description of the actual patterns.
The three key departures from expected utility
Prospect theory formalises three specific departures.
Reference dependence. Utility depends not on absolute wealth levels but on gains and losses relative to a reference point. A person with $100,000 who loses $5,000 experiences the loss subjectively very differently from a person with $100,000 who has just gained $5,000 from a starting position of $95,000. In classical theory, both should evaluate their $95,000 position identically. In prospect theory, the reference point matters enormously.
Loss aversion. Losses are felt subjectively more strongly than equivalent gains. The ratio is roughly 2:1 based on Kahneman and Tversky's original work and much subsequent replication. Losing $100 hurts approximately twice as much as gaining $100 feels good.
Probability weighting. Humans do not multiply outcomes by their objective probabilities. Small probabilities are systematically overweighted (which is why lottery tickets and insurance policies are both attractive), while moderate-to-high probabilities are somewhat underweighted (which is why "sure things" don't feel as attractive as their expected value would suggest).
The specific investment implications
Prospect theory's departures from expected utility explain many well-documented investor behaviours.
The disposition effect (cutting winners early, holding losers too long) follows directly from loss aversion and reference dependence. Investors evaluate positions relative to their cost basis (reference point). Selling a winner locks in a certain gain that is felt as positive utility. Selling a loser locks in a loss that activates the 2x loss aversion weighting. The asymmetric feeling produces the asymmetric behaviour.
Insurance and lottery ticket combinations arise from probability weighting. The same person who buys life insurance (overweighting the small probability of catastrophic loss) also plays the lottery (overweighting the small probability of extraordinary gain). Classical theory struggles to explain both behaviours simultaneously in the same person; prospect theory predicts both from a single probability-weighting function.
Portfolio underdiversification for lottery-like assets. Investors systematically hold more of speculative assets with large potential upside than expected-utility theory would predict. The small probability of enormous gain is overweighted in the mental valuation, producing overallocation to these assets.
Underdiversification of employer stock. Employees hold more employer stock in their retirement accounts than diversification would suggest. Multiple factors contribute, but reference dependence is one — the emotional connection to the employer creates a reference point that anchors evaluation.
The framing effect
A specific and consequential feature of prospect theory is the framing effect. The same objective outcome can be described in different ways that produce different subjective evaluations.
The classic example: subjects are told that a disease is expected to kill 600 people and asked to choose between two treatments. In one framing, the choice is between "Program A: 200 people will be saved" and "Program B: 1/3 probability that 600 people will be saved and 2/3 probability that no one will be saved." Most subjects choose Program A (the certain gain). In another framing, the choice is between "Program A: 400 people will die" and "Program B: 1/3 probability that no one will die and 2/3 probability that 600 people will die." Most subjects choose Program B (the gamble to avoid the certain loss).
The two framings describe mathematically identical outcomes. But the first frames outcomes as gains (relative to a reference of everyone dying); the second frames them as losses (relative to a reference of no one dying). Loss aversion makes the gamble more attractive when outcomes are framed as losses. The same person who chose A in the first framing chooses B in the second.
The framing effect operates constantly in investment decisions. "The stock is down 20% from where I bought it" and "the stock is up 40% from its 52-week low" describe the same current price differently, and produce different subjective evaluations. The framing of any market information influences the resulting decision, whether or not the receiver is aware of the framing.
The behavioural corollaries
Multiple specific behaviours follow from prospect theory's structure.
Status quo bias. When a change would potentially produce gains or losses relative to the current position, loss aversion makes people preferentially avoid the change. This produces excessive inertia in portfolios — investors don't rebalance, don't change allocations, don't add new investments, because any change carries potential for regret.
Endowment effect. Ownership of an asset produces a higher subjective value than the same asset would be worth to a potential buyer. Investors who own a stock tend to require a higher price to sell than they would offer to buy the same stock if they didn't own it. The effect is a specific consequence of loss aversion applied to the potential loss of the owned asset.
Mental accounting. Investors mentally partition their portfolio into separate accounts (retirement, kids' education, house down payment) and treat them as if they were independent, when in reality all funds are fungible. The mental accounting produces specific investment behaviours that classical theory would consider inefficient.
What can be done about prospect-theory patterns
None of these patterns can be fully eliminated through awareness. Prospect theory describes how humans actually make decisions, and being aware of the descriptions does not turn a human into a classical expected-utility maximiser. But three practices reduce the specific costs.
Frame at the portfolio level rather than the position level. Loss aversion is triggered more strongly by individual position losses than by portfolio-level losses. Evaluating positions in the context of the portfolio reduces the trigger.
Pre-commit to systematic rules. Written policy statements about position sizing, rebalancing, and exit criteria remove specific decisions from prospect-theory influence. The rule was decided when the reference point and loss-aversion weightings were not activated; honouring the rule protects against their activation during specific market events.
Recognise the framing effect explicitly. When receiving investment information, ask what the reference point is and how the framing might be different. This does not eliminate the framing effect but partially calibrates evaluations.
The rule to internalise
Prospect theory is not a description of how people should make decisions; it is a description of how they actually do make decisions. Understanding the specific ways human decision-making departs from classical rationality explains most of what looks like irrational investor behaviour but is really the predictable output of a specific psychological system. The system cannot be turned off through awareness, but its costs can be reduced through practices that limit its influence on consequential decisions. This is one of the most important intellectual frameworks in behavioural finance and deserves more attention than casual retail commentary typically gives it.
Educational content only. Not investment advice.