The mechanics of Prospect Theory
Kahneman and Tversky found that people evaluate gains and losses relative to a reference point (usually the price they paid), not in absolute terms. The value function has two key properties: it is concave for gains (diminishing marginal pleasure as gains grow) and convex for losses (diminishing marginal pain as losses grow), and the slope is steeper for losses than for gains — approximately 2:1 in most studies. This means the pain of losing £100 is roughly equivalent to the pleasure of gaining £200. People therefore make irrational choices to avoid losses: they will take a certain small loss over a gamble with better expected value (to avoid the pain of certainty), and choose a certain gain over a positive-expected-value gamble (preferring the certainty of a gain even when the gamble offers higher expected value).
The disposition effect
Loss aversion directly causes the "disposition effect" — the tendency to sell winning investments too early and hold losing investments too long. Selling a winner locks in the gain, providing a burst of positive emotion. Holding a loser avoids the pain of formally realising the loss — as long as the position is open, there is still hope of recovery, and the loss remains "on paper" (psychologically less real). This produces precisely the opposite of optimal investing: cutting winners short and letting losers run. The irony is that investors who exhibit the disposition effect simultaneously congratulate themselves on their "gains" while rationalising their "temporary" losses — both the gains and losses reflect the same position in time.
Loss aversion in everyday financial life
Beyond investing, loss aversion shows up pervasively in financial decisions. People refuse to switch energy providers, insurance policies, or bank accounts even when switching would save significant money — the perceived risk of a worse outcome feels more real than the expected gain. People keep cash in low-interest accounts rather than moving it to higher-yield savings because moving it feels like a commitment that might be regretted. People decline to pay off a credit card balance with their savings because losing the savings feels worse than gaining the interest saving. In each case, the expected-value-maximising choice is clear, but loss aversion distorts the decision.
“The question is not how to eliminate loss aversion — it is wired into human psychology. The question is how to build financial systems that prevent it from making your decisions for you.”
What this means for you
The most practical defence against loss aversion is pre-commitment: making investment decisions in advance, through rules, before the emotional response to losses can operate. An investment policy statement (IPS) specifies what you own, why, your target allocation, and under what conditions you would sell — removing discretionary decisions in the heat of market stress. Dollar-cost averaging and automatic rebalancing similarly remove the temptation to act on loss-averse impulses. When you feel a strong urge to sell a falling asset, ask: if this position were at zero and I had the cash in hand, would I choose to invest it here? If yes, hold. If no, that is an honest signal rather than loss aversion talking.