Two people each learn about a medical treatment. One is told the treatment has a 90% survival rate; the other is told it has a 10% mortality rate. Both statements are identical — but people evaluating the framing-as-survival rate the treatment significantly more favorably than the framing-as-mortality rate. Expected utility theory predicts no difference; people's actual responses show a large, consistent difference. This framing effect — and dozens of other systematic departures from rational expected utility maximization — motivated Daniel Kahneman and Amos Tversky to develop prospect theory, which earned Kahneman the 2002 Nobel Prize in Economics.
The setup
Prospect theory is a descriptive model of decision-making under risk that matches observed human behavior more accurately than expected utility theory. It rests on four key features:
1. Reference dependence: people evaluate outcomes as gains or losses relative to a reference point (usually the status quo or an expectation), not as absolute wealth levels. Gaining $100 from a baseline of $1,000 feels different from arriving at $1,100 directly.
2. Loss aversion: losses feel roughly twice as painful as equivalent gains feel pleasurable. Losing $500 hurts more than winning $500 feels good. This asymmetry is the most empirically robust finding in behavioral economics — NBER research on loss aversion documents it across dozens of contexts from financial markets to labor supply.
3. Diminishing sensitivity: the value function is concave for gains (each additional dollar of gain matters less) and convex for losses (each additional dollar of loss hurts less at the margin). A $100 gain from $0 feels larger than a $100 gain from $1,000; a $100 loss from $0 feels worse than a $100 loss from $1,000.
4. Probability weighting: people do not use objective probabilities in their decisions. They overweight small probabilities (explaining insurance and lottery demand simultaneously — both involve overweighted small probabilities of large losses or gains) and underweight large probabilities.
What happens — and why
The S-shaped value function of prospect theory generates several well-documented behavioral patterns:
Disposition effect in investing: investors hold losing positions too long (hoping to avoid locking in a loss) and sell winning positions too early (eager to lock in a gain). The SEC's investor education research documents this pattern in retail investor portfolios — a direct consequence of loss aversion at the reference point of the original purchase price.
Endowment effect: people demand more to give up something they own than they would pay to acquire it — the loss of giving something up looms larger than the gain of acquiring it. Behavioral research published through NBER documents this in auctions, negotiations, and market experiments.
Status quo bias: the reference point creates inertia — departing from the status quo can feel like a loss even when change would improve outcomes. This is related to the endowment effect and explains default-option persistence in retirement savings, insurance enrollment, and program participation.
Where you see it in the wild
Financial advisors, retirement plan designers, and insurance marketers all implicitly apply prospect theory. The CFPB's financial product disclosure research shows that framing retirement savings as avoiding future losses ("don't run short of money in retirement") increases contribution rates compared to framing as gains — loss aversion makes loss-framing more motivating.
Why it matters
Prospect theory is the most influential descriptive model of decision-making under risk. It doesn't prescribe how decisions should be made — it describes how they are made. Understanding its predictions allows financial product designers, policymakers, and advisors to structure choices that work with people's actual psychology rather than assuming it away. It also explains puzzles that classical utility theory cannot: why simultaneous insurance and lottery purchase is rational under prospect theory but paradoxical under expected utility.
