The Money You've Already Spent Has Nothing to Do With Your Next Decision

In 2003, the British and French governments were still pumping money into the Concorde supersonic jet program. They had each spent hundreds of millions across three decades, the planes were hemorrhaging operating losses, and every credible forecast said the economics would never work. The decision to keep flying was not made on the merits of the next dollar spent. It was made because stopping meant admitting the previous dollars were gone. They kept paying to avoid the pain of writing off what they had already lost. Economists now call this the Concorde fallacy, and it is the clearest large-scale demonstration of a mistake most people repeat every day on a smaller scale.¹

What a sunk cost actually is

A sunk cost is a cost you have already paid and cannot recover, whatever you decide next. The $90 concert ticket you bought for tonight is sunk the moment you bought it. The $40,000 you poured into a kitchen renovation that stalled is sunk. The three years you have already spent in a graduate program you have come to dread are sunk. None of those figures can be retrieved by any choice you make going forward, which means none of them carry any information about which choice is now better.

The rule that follows from this is blunt. As Pierre Lemieux states it at the Library of Economics and Liberty, "a rational decision-maker will not include sunk costs in his decisions. Since sunk costs are unrecoverable by definition, they have nothing to do with decisions made now for the future."¹ The only costs and benefits that matter to a forward-looking decision are the marginal costs and benefits of each path from here: what each option will cost you going forward, and what each option will return.⁴

This connects directly to how economists think about opportunity cost. A dollar already spent does not appear in any of your available futures. No choice you make can bring it back or redeploy it. It has dropped out of the relevant math entirely.

The question that cuts through it

Let's start with the practical tool before we get into why the fallacy is so hard to shake. There is one question that forces the issue:

If I were starting fresh today, with no money or time already in this, would I still choose it?

That reframe strips the history out and leaves only what actually matters: the forward path. If a project, a stock position, or a commitment is not worth starting fresh today on its own merits, then continuing it only because of what you have already put in is the fallacy at work. The discipline is to keep your eyes on what comes next and refuse to steer by what is already behind you.

Two examples at different scales

Shift to the small one first. You bought a stock at $80. It is now trading at $50, and your honest read of the company is mediocre. You find yourself thinking, "I will sell once it gets back to $80." That anchor is the purchase price, and the purchase price is a sunk cost. The market does not know or care what you paid. The $30 loss is already real whether you hold or sell.⁵ The only live question is whether $50 of capital is better left in this stock or moved to something with better forward prospects. Run the fresh-start question: would you buy this stock today at $50? If the answer is no, the only thing keeping you in it is a sunk cost.

Now the large one. A company has spent $300 million developing a product, and new market data says it is unlikely to succeed. The instinct in that boardroom is to say they have invested too much to stop. But the $300 million is spent either way. The real decision is whether the additional capital required to finish will earn a return that beats putting it somewhere else. If finishing costs another $100 million to chase a product that will lose money, the sunk $300 million is not a reason to spend the $100 million. It is the price of finding out the product does not work, and that information is itself valuable if you act on it.⁴ Governments and large firms have burned billions completing doomed projects on exactly this logic. The fallacy scales.

Why letting go feels wrong

If the rule is this clean, why does nearly everyone violate it? Because human beings are wired to hate losses more than they love equivalent gains. Daniel Kahneman, Amos Tversky, and Richard Thaler documented this asymmetry precisely, and it sits at the center of prospect theory: losing $100 registers as roughly twice as painful as gaining $100 feels good.⁶ Abandoning a sunk investment forces you to formally register a loss, and that registration hurts. Holding on lets you defer the pain, even when holding on guarantees a larger loss down the road.

Thaler, who won the 2017 Nobel Prize in Economic Sciences for this line of work, showed that real people (he called them "Humans" to distinguish them from the perfectly rational "Econs" of textbook models) systematically let sunk costs drive their choices.² The Concorde governments kept the planes flying. Poker players double down to "get even." Investors hold losing positions for years. The fallacy is not stupidity. It is a predictable feature of how the human mind accounts for loss, which means naming it is the first and most important defense against it.³

Where the model breaks down

Now shift to the limits, because the rule "ignore sunk costs" misfires when applied carelessly. Three real costs are easy to mislabel as sunk when they are not.

Reputation and trust are future costs. A firm that walks away from a half-finished commitment to customers may rationally write off the spending. But the damage to its reputation affects future sales, future hires, and future partnerships. That is a genuine forward cost and belongs in the decision.

Contracts create future obligations. Walking away from a project may trigger penalties or breach-of-contract claims. Those are real future cash flows. The sunk part is what you have already spent. The contractual exposure is what you still owe.

Sometimes the spending bought information. Money spent learning that a path does not work is not wasted in the full sense: it purchased knowledge that sharpens the next decision. The cash is sunk, but the lesson is a forward-carrying asset. Treating hard experience as pure loss can push you to ignore what you have actually learned.

None of these undercut the core rule. They sharpen it. The discipline is not to pretend the past never happened. It is to count only the costs and benefits that still lie ahead, and to be honest with yourself about which ones those really are.

The next time you feel it

Overall, the sunk cost fallacy is one of the most expensive mental habits in finance precisely because it feels like responsibility. Holding on, pushing through, not "wasting" what you have put in: these feel like virtues. What they actually are is the future paying for decisions the past already made.

The next time you feel the pull of "I have put too much into this to stop," run the fresh-start question. If you would not start it today with clear eyes and no history, the money you have already spent is not a reason to continue. It is just the price of finding that out. Decide from here.