The stock has fallen 40%. The thesis that justified buying it has been invalidated. Every rational framework says it’s time to sell. But you don’t sell. Instead, you check it every morning, you read every earnings call transcript hoping for something to justify your conviction, and you tell yourself you’ll sell when it gets back to breakeven.
You’re not stupid. You’re experiencing the sunk cost fallacy — one of the most psychologically powerful biases in all of human decision-making.
What Is the Sunk Cost Fallacy?
A sunk cost is any cost — money, time, or energy — that has already been spent and cannot be recovered. The rational economic principle is that sunk costs should be completely irrelevant to forward-looking decisions. The sunk cost fallacy is the tendency to continue with a course of action because of the resources previously invested — even when that course of action no longer makes rational sense.
📊 What Research Shows
Research by Hal Arkes and Catherine Blumer (1985) demonstrated that people will continue investing in a losing project specifically because they have already invested significantly in it — even when rational analysis clearly indicates abandonment would be optimal. This has been replicated hundreds of times across cultures and contexts.
Why Sunk Costs Are So Psychologically Powerful in Investing
The sunk cost fallacy in investing is supercharged by loss aversion. Kahneman and Tversky demonstrated that losses feel approximately 2.5 times more painful than equivalent gains feel pleasurable. Selling a losing stock means converting an unrealized loss (which your brain can minimize by telling itself it might recover) into a realized loss (which is final, concrete, and psychologically devastating).
The Breakeven Trap
The most common manifestation: “I’ll sell when it gets back to what I paid.”
This reasoning contains two compounding errors. First: the stock doesn’t know what you paid for it. Whether the stock is worth holding has nothing to do with its price relative to your purchase price. Second: the opportunity cost is ignored. Every day you hold a losing position waiting for breakeven, that capital could be deployed elsewhere.
🔑 Key Takeaway
Ask yourself: if you didn’t already own this stock, would you buy it today at the current price? If the honest answer is no, you’re being held by the sunk cost fallacy. The rational question is never ‘how do I get back to breakeven?’ It’s always ‘what is the best use of this capital from this moment forward?’
Warren Buffett’s Rule
Buffett famously said: “When you find yourself in a hole, stop digging.” His approach to sunk costs is almost surgical: the past is gone. The only question is what the expected return is from this day forward, compared to alternative uses of capital.
How to Build a Sunk Cost Escape System
Pre-set exit criteria
Before buying any position, define the conditions that would invalidate your thesis. Write them down. If those conditions are met, the decision to sell has already been made — you’re just executing it.
Anonymize your positions
Monthly, review your portfolio as a list of ticker symbols with no reference to your cost basis. Ask: based purely on the current price and my best assessment of future value, would I hold, buy more, or sell each position? Your cost basis is a sunk cost.
Tax-loss harvesting
In taxable accounts, realized losses create tax benefits. Reframing a losing sale as “generating a tax asset” can reduce the psychological pain of crystallizing the loss — making it easier to escape the sunk cost trap.
📚 Recommended Reading
The research on sunk costs and financial decision-making is covered brilliantly in Dan Ariely’s Predictably Irrational — which shows that our financial irrationality is not random but systematic and predictable.
Holding a losing investment is not loyalty. It’s not patience. It’s not discipline. It’s the brain’s loss-avoidance mechanism masquerading as investment strategy.
Continue the Cognitive Bias Mastery Series.
Written by
Monteiro
Investor · Behavioral Finance Writer · 20+ Years of Market Experience
life enthusiast, self-proclaimed scientist, philosopher, ...
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