Sunk-cost fallacy

The sunk-cost fallacy is the tendency to throw good money after bad, continuing to invest in a losing position because you have already invested so much. The $10,000 you paid for a stock is gone, whether you hold or sell. Yet the sunk cost psychologically “anchors” you, making you reluctant to sell and accept the loss. Rationally, only future prospects matter; the past cost should be irrelevant.

Why it is a fallacy

The fundamental insight is simple: a sunk cost is a cost that has been incurred and cannot be recovered. Your purchase price for a stock is sunk. Whether you bought at $100 or $50 does not matter to the future decision. Only two things matter:

1. Current value: what is the stock worth now?
2. Future prospects: what will it be worth in the future?

If you would not buy the stock at its current price, you should sell it. The fact that you paid more in the past is irrelevant.

Yet, investors constantly violate this principle. They hold losing stocks because “I cannot sell at a loss; I have to break even.” This is the sunk-cost fallacy.

The mechanism

The fallacy arises from several psychological forces:

Loss aversion. Realizing a loss feels painful. The sunk cost is a reminder that you made a mistake. Selling crystallizes the loss and forces you to confront the failure. Holding preserves the option that the loss might be recovered.

Mental accounting. You mentally earmark the money for a “stock investment account.” The account is down 30%. To “break even” on the account, you hold or even add to it. You do not view it as part of your total wealth; you view it as a separate entity that must be redeemed.

Regret aversion. Selling locks in the regret of having made a bad purchase. Holding preserves the hope that the bad purchase will be validated. The prospect of regret if you sell and the stock then recovers is unbearable.

Sunk-cost fallacy in practice

Averaging down. An investor bought a stock at $100 and it fell to $70. Rather than accept the loss, she buys more at $70, averaging her cost to $85. If the stock falls to $50, she buys again. This is classic sunk-cost fallacy: throwing good money after bad in an attempt to reduce the psychological pain of the loss.

Holding through fundamental deterioration. A company’s fundamentals have deteriorated, but the investor holds because “I have a lot of money in this.” The $10,000 she paid is sunk. The relevant question is: at $70 per share, is the stock still a good investment? If not, she should sell.

Corporate projects. Businesses fall victim too. A company has invested $50M in a project and asks: should we invest another $10M to finish it? If they analyze only the remaining investment and the marginal return, they might proceed. But if the marginal return is negative, the prior $50M is sunk and irrelevant. They should abandon the project.

Sunk-cost fallacy and confirmation bias

Sunk-cost fallacy and confirmation bias amplify each other. An investor holding a losing stock for sunk-cost reasons will seek confirming evidence that it will recover. This confirmation-seeking reinforces the decision to hold.

Sunk-cost fallacy vs. loss aversion

Loss aversion is the asymmetric pain of losses. Sunk-cost fallacy is the logical error of treating past costs as relevant. They are related but distinct. Loss aversion explains why the pain is there; sunk-cost fallacy is the specific error of using that pain to make decisions.

Defenses against sunk-cost fallacy

• Ask: if I did not own this stock, would I buy it at today’s price? This is the fundamental test. If the answer is no, sell it. The past price is irrelevant.
• Track your purchase price separately from your decision. Write down the stock’s current price and your forecast for its future price. Do not even look at what you paid. Make the decision based on current value and future prospects.
• Set a loss limit before you invest. Before buying a stock, decide: “I will sell if it falls 25% from the purchase price.” This pre-commitment prevents sunk-cost reasoning later.
• View losses as tuition. Treat investment losses as the cost of learning, not as failures to be recovered. This reframing reduces the emotional pain and allows rational decisions.
• Use a mechanical stop-loss. A stop-loss order sells automatically if the price falls below a threshold. This removes the emotional sunk-cost reasoning from the decision.
• Separate past from future. Say to yourself: “The money I paid is gone, whether I hold or sell. What should I do with my money going forward?” This frame change helps resist sunk-cost reasoning.

See also