The Sunk Cost Fallacy

The Fifteen Dollars You Can't Get Back

You paid $15 for a movie ticket. Thirty minutes in, you know with the cold certainty of a closing theorem that this film is terrible. Not quirky-bad, not so-bad-it's-good — just bad. The acting is wooden, the plot is incoherent, and the person next to you is eating popcorn with the acoustic profile of a demolition site. Do you leave?

Most people stay. And when you ask them why, they say something like: "Well, I already paid fifteen dollars." As if sitting through another ninety minutes of cinematic suffering will somehow un-spend the money. As if the ticket price is a hole that can only be filled by enduring the rest of the film.

This is one of the most common reasoning errors humans make, and it has a name: the sunk cost fallacy. It's the tendency to continue an endeavor once an investment of money, time, or effort has been made — not because continuing is worthwhile, but because abandoning it would mean "wasting" what's already been spent.

Here's the thing mathematicians know that most people don't: the fifteen dollars is gone. It's gone if you stay and it's gone if you leave. It's gone if you set off fireworks in the lobby. The money has been converted, irrevocably, into a non-refundable ticket. The only question that matters now is: what do I do with my next ninety minutes?

The rational rule is almost offensively simple: only future costs and future benefits should influence current decisions. Past expenditures, once irrecoverable, are irrelevant. They're sunk. They've drowned. Stop trying to resuscitate them.

And yet. And yet. Humans are spectacularly, reliably, magnificently terrible at this.

The Experiment That Proved It

In 1985, psychologists Hal Arkes and Catherine Blumer ran a beautifully simple experiment at the Ohio University theater.1 At the start of the season, they randomly gave ticket buyers one of three deals: some paid full price, some got a modest discount, and some got a steep discount. Then they watched who actually showed up.

The result was exactly what sunk cost theory predicts and exactly what rational choice theory says shouldn't happen: people who paid full price attended significantly more shows than those who got discounts — even when the shows were, by all critical accounts, mediocre. The amount you paid for the ticket was influencing whether you used it, even though the payment was already made and nonrefundable.

Think about what this means. A rational agent would decide whether to attend Tuesday night's performance based on: How good is the show? What's the weather like? What else could I do tonight? The purchase price of the ticket, already paid and gone, should carry precisely zero weight. But it doesn't carry zero weight. It carries enough weight to drag people through freezing Ohio evenings to watch performances they'd rather skip.

Arkes and Blumer ran a whole series of these experiments. In one, they asked subjects to imagine they'd bought a $100 ticket to a ski trip in Michigan and a $50 ticket to a ski trip in Wisconsin. Then they learned, whoops, the trips are the same weekend. Which do you attend? Most people chose the $100 trip — even when told they'd enjoy the Wisconsin trip more. They were choosing more misery to honor the larger sunk cost.2

How Good Are You at Ignoring Sunk Costs?

Let's find out. Here are six scenarios, each one a little more tempting than the last. In each case, you've already spent something — money, time, effort — and now you face a choice: push forward or cut your losses. The rational answer depends only on what happens next, not what already happened.

The Concorde: Supersonic Sunk Costs

If the movie-ticket example seems too trivial to worry about, consider the Concorde.

The Concorde was a marvel of engineering — a supersonic passenger jet that could cross the Atlantic in three and a half hours — and an absolute catastrophe of economics. By the early 1970s, both the British and French governments knew the aircraft would never be commercially viable. The development costs had spiraled. Airlines weren't placing orders. The math was clear.3

They kept funding it anyway. For decades.

Why? Because they'd already spent billions. Walking away would mean "wasting" all that money. So instead, they spent billions more, which is the sunk cost fallacy's favorite punchline: the attempt to avoid wasting money guarantees you waste more money.

Biologists actually call this the "Concorde fallacy" — it shows up in animal behavior too, when a bird continues incubating a doomed egg because it's already invested weeks of sitting.4 Evolution, apparently, didn't fully solve this problem either.

The same logic infected the Vietnam War. By the late 1960s, the strategic calculus was grim: the war was unwinnable at any cost Americans were prepared to pay. But the political argument kept circling back: "We can't let those soldiers have died in vain." The deaths of past soldiers — tragic, irrecoverable — were being used to justify sending more soldiers to die. The sunk cost wasn't just money anymore. It was lives.5

Barry Staw formalized this in 1976 as escalation of commitment: once people have made a decision and invested in it, they tend to increase their investment even as evidence mounts that the decision was wrong.6 The investment creates a psychological gravity well. The more you put in, the harder it is to climb out.

Why We Fall For It

The sunk cost fallacy isn't random irrationality. It's the predictable output of several deeply wired psychological mechanisms.

Loss Aversion

Daniel Kahneman and Amos Tversky showed that losses hurt roughly twice as much as equivalent gains feel good.7 When you walk out of that bad movie, you're not just "deciding to do something else with your evening." Psychologically, you are realizing a loss. The $15 transitions from "investment in tonight's entertainment" to "money I wasted." As long as you sit in the theater, the loss remains theoretical, Schrödinger's expenditure. The moment you leave, the cat is definitely dead.

So you stay. Not because staying is enjoyable, but because leaving would force you to confront the loss. You're paying ninety minutes of boredom to avoid thirty seconds of regret.

The Consistency Trap

Humans have a deep need to appear consistent — to themselves and others. You chose this movie. You told your friends it looked good. Walking out is an admission of error. And our brains would rather sustain a losing position than admit the position was wrong in the first place.

This is particularly devastating in organizations. A CEO who championed a failing product launch will keep pouring resources into it not because the numbers justify continuation, but because cancellation is a confession. The board approved funding; reversing course means admitting the board was wrong. And so the money flows.

The "Waste Not" Heuristic

There's also a simpler, almost folksy explanation: we were raised not to waste things. Finish your dinner. Wear the shoes until they fall apart. Use every last drop. This is a fine heuristic when applied to resources that still have value. But it misfires catastrophically when applied to irrecoverable costs. You can't "use up" a sunk cost. It's already used. Sitting through the bad movie doesn't extract any remaining value from the ticket — the ticket's value was the option to see the movie, and you've exercised it. What you learned is that the movie is bad. That's all the value there is.

The Investment Simulator

Here's where it gets really painful. The sunk cost fallacy doesn't just make you sit through bad movies — it makes businesses pour millions into doomed projects, governments fund bridges to nowhere, and investors hold collapsing stocks. Let's see it in action.

In this simulator, you're managing a software project. Each round, the project has a probability of eventually succeeding. You can invest another $100K, or you can abandon and cut your losses. A rational agent evaluates only the expected future value. A sunk-cost thinker? Well, let's find out.

When Sunk Costs Aren't Really Sunk

Before you go around triumphantly walking out of every movie and quitting every project, a caveat: not everything that looks like a sunk cost is actually sunk.

Suppose you've spent three years learning to play the piano. You're not great, and you're not sure you want to continue. Is the three years a sunk cost? In one sense, yes — you can't get those hours back. But here's the thing: those three years produced something that still exists. You can read music. You have finger dexterity. You understand rhythm and harmony. That's not a sunk cost — that's an acquired asset. The question isn't "should I honor my past investment?" but "given the skills I now have, is continuing worthwhile?"

See the difference? The sunk cost framing says: "I've put in three years, so I have to continue." The rational framing says: "I've acquired skills that make continuing more valuable than starting from scratch at something else." Same decision, entirely different reasoning. One is fallacious, the other is sound.

Similarly, reputation can look like a sunk cost but isn't, exactly. If you've built a track record in a field, that reputation has future value. Staying in the field to leverage that reputation isn't sunk cost reasoning — it's recognizing a forward-looking advantage.

The test is simple: does the past investment create something that still provides future value? If yes, it's an acquired asset, not a sunk cost. If no — if the money or time is simply gone, with nothing to show for it but the fact that it was spent — then it's sunk, and you should ignore it.

The Formula for Forward-Looking Decisions

If we want to be precise — and this is a math book, so we do — the rational decision rule looks like this:

Notice what's absent: any variable for "amount already spent." The formula doesn't care. It can't care, because the amount already spent is a constant that doesn't change with your choice of action. It adds the same number to every option, so it cancels out of the comparison. That's not a philosophical position — it's algebra.

The sunk cost thinker, by contrast, is implicitly optimizing a different function — one that includes a term like −λ · (sunk cost) · I(abandon), where λ is some psychological pain coefficient and I(abandon) is an indicator that fires when you quit. They're not maximizing future value. They're minimizing psychological discomfort. And those are very, very different objectives.8

Learning to Leave the Theater

The sunk cost fallacy is everywhere once you start looking. It's the gym membership you keep paying for because you "already committed." It's the book you force yourself to finish because you're halfway through. It's the career you stay in because you "already invested ten years." It's the war you keep fighting because you "already lost so many soldiers."

And it scales. A bad movie costs you ninety minutes. A bad business decision costs millions. A bad policy, sustained by the weight of sunk costs, can cost lives.

The cure is philosophically simple but psychologically brutal: you must learn to evaluate every decision as if you just arrived. As if you have no history with this project, this movie, this relationship. You must ask only: given where I am now, and what I know now, what is the best use of my future resources?

That's hard. It requires you to admit past mistakes rather than trying to redeem them through further investment. It requires you to accept that some money, some time, some effort is simply gone — and that the best response to a loss is not to compound it.

Mathematicians have a phrase that applies beautifully here: without loss of generality. It means: we can simplify the problem without actually losing anything important. That's exactly what ignoring sunk costs does. It simplifies your decision by removing information that, however emotionally salient, is mathematically irrelevant.

The $15 is gone. The question is what you do with the next ninety minutes. And the answer, if you're being rational about it, is: literally anything other than sitting in that terrible movie.