What Is Prospect Theory?
Prospect theory, published by Daniel Kahneman and Amos Tversky in 1979, overturned the classical economic assumption that people make rational decisions to maximize expected utility. The theory demonstrates two key insights: first, people evaluate outcomes relative to a reference point (usually the status quo) rather than in absolute terms; second, losses loom roughly twice as large as equivalent gains, a phenomenon called loss aversion. Kahneman received the Nobel Prize in Economics in 2002 largely for this work.
The Value Function and Investment Implications
The prospect theory value function is S-shaped: concave for gains (diminishing sensitivity) and convex for losses (increasing risk-seeking). This explains why investors often reject a fair gamble with a 50% chance of gaining $1,000 and a 50% chance of losing $1,000, because the psychological weight of the potential loss exceeds that of the potential gain. In practice, loss aversion causes investors to hold losing stocks too long (hoping to break even) and sell winners too quickly (locking in the pleasure of a gain), directly producing the disposition effect.
Key Considerations
Awareness of prospect theory is the first step toward better decisions, but awareness alone is insufficient. Practical countermeasures include pre-committing to investment rules (such as automatic rebalancing), framing portfolio performance in terms of long-term cumulative returns rather than daily fluctuations, and checking your portfolio less frequently. Research by Benartzi and Thaler (1995) showed that investors who reviewed returns annually took on significantly more equity risk than those who checked monthly, because less frequent evaluation reduces exposure to short-term losses.