Prospect Theory: Why Losing $20 Hurts More Than Finding $20 Feels Good

In 1979, psychologists Daniel Kahneman and Amos Tversky published a paper that permanently changed economics. They proved that people do not treat gains and losses equally. Finding a $20 bill produces far less happiness than losing a $20 bill produces pain — even though the dollar amounts are identical. This discovery, called Prospect Theory, earned Kahneman the Nobel Prize in Economics in 2002.

In this lesson you will: • Understand loss aversion and why the loss coefficient lambda is approximately 2.25 • Identify the reference point effect and how it shifts your perception of gains and losses • Analyze the S-shaped value function and what its shape reveals about human psychology • Apply Prospect Theory to real decisions in investing, marketing, and healthcare messaging

Kahneman and Tversky ran hundreds of experiments on university students. In one classic version they posed two problems back-to-back. Problem 1: Would you rather have (A) a guaranteed $900, or (B) a 90% chance to win $1,000? Most people chose A — the sure thing — even though the expected value of B is identical ($1,000 × 0.90 = $900). People dislike risk when chasing gains. Problem 2: Would you rather have (A) a guaranteed loss of $900, or (B) a 90% chance to lose $1,000? Now most people chose B — the gamble — even though the expected values are again the same. People take MORE risk to avoid a sure loss. This reversal — risk-averse for gains, risk-seeking for losses — could not be explained by classical economics. It demanded a new model.

Prospect Theory describes how we subjectively feel about outcomes using a value function with three key features. 1. Reference point: All outcomes are evaluated as gains or losses RELATIVE to a starting expectation — not in absolute terms. If you expected a $500 bonus and received $300, you feel a $200 loss even though you gained $300 in real money. 2. Diminishing sensitivity: Each extra dollar produces less psychological impact than the one before it. Going from $0 to $100 feels far better than going from $900 to $1,000 — both are +$100, but the first feels bigger. The curve flattens out the further you move from the reference point in either direction. 3. Loss aversion: The loss side of the curve is steeper than the gain side. Kahneman and Tversky measured the loss aversion coefficient (lambda) at approximately 2.25. Losing $100 feels about 2.25 times as bad as gaining $100 feels good.

Marketing: Retailers frame savings as avoiding a surcharge rather than getting a discount. 'Cash price: $2.00 — credit card surcharge: $0.10' feels better than 'Credit price: $2.10 — cash discount: $0.10' because the first frames credit as incurring a loss, motivating customers to pay cash. Investing: Loss aversion explains why investors hold losing stocks too long — refusing to realize a loss — and sell winning stocks too early to lock in a gain. Economist Terrance Odean documented this disposition effect across 10,000 brokerage accounts in 1998. Healthcare: Telling patients they have a '10% chance of dying' produces more procedure refusals than telling them they have a '90% chance of surviving' — identical statistics, opposite frames. Physicians who understand this can communicate risk more clearly and ethically.