The sunk cost fallacy is the pull to keep spending money, time or effort on something because of what you've already put in — even when none of it can be recovered and stopping would leave you better off.
Named and experimentally demonstrated by Hal Arkes and Catherine Blumer in 1985. The underlying concept of a sunk cost is older, and comes from economics rather than psychology.
// Where it sits
The only question that isn't contaminated: would I choose this today, arriving fresh?
What it really means
Arkes and Blumer's 1985 study is elegant because the participants never knew they were in it. People buying a season ticket to a university theatre were handed, at random, either the full $15 price, a $13 price, or an $8 one. Same seats, same plays, same season — the only difference was a discount nobody had asked for and nobody could have earned.
Over the first half of the season, the people who had paid full price showed up more often. Not because they enjoyed theatre more; they'd been assigned their price by chance. Because $15 was a larger amount of money to have thrown away, and going to the play was the only way to stop it being thrown away.
The part worth being careful about is that not every persistence is this fallacy. Past spending sometimes carries genuine information about the future — a car with £2,000 of recent repairs really may be more reliable now. The test isn't whether you spent; it's whether the spending is doing any forward-looking work in your reasoning. When the entire argument for continuing is the size of the bill so far, you're not making a decision. You're paying an old one interest.
Where it comes from
Economics says a rational decision-maker treats sunk costs as if they don't exist. The fallacy is that we can't.
Myths & misconceptions
Any time you carry on with something you've paid for, that's the sunk cost fallacy.
It's only the fallacy when the past cost is unrecoverable AND the forward-looking case says stop. Finishing a course because the remaining classes are genuinely worth your time is just a good decision.
It's about money.
Money is only the easiest version to measure. Time, effort and public commitment produce the same pull — and Arkes and Blumer traced it to not wanting to appear wasteful rather than to the arithmetic itself.
Compare & contrast
Sunk cost fallacy vs loss aversion
Loss aversion is why quitting hurts — you feel the write-off as a loss. The sunk cost fallacy is what you then do about it: keep spending to postpone booking that loss. One is the engine, the other is the behaviour it drives.
| // | Sunk cost fallacy | Loss aversion |
|---|---|---|
| Is | A behaviour | A feeling |
| Looks | Backward | At the reference point |
| Produces | Continued spending | Reluctance to realise |
| Named | 1985 | 1979 |
How it connects
- Loss Aversionthe mechanism underneath — quitting converts a paper loss into a real one.
- Ikea Effectthe mirror case: sunk cost binds you to the unfinished, the IKEA effect to the finished.
Tell it apart
Questions people ask
- What is the sunk cost fallacy?
- Continuing something because of what it already cost. You're forty minutes into a bad film at the cinema; the ticket money is gone whether you stay or leave, but you sit through another ninety minutes 'because I paid for it' — and lose the evening on top of the ticket.
- Is it always irrational to consider what you've already spent?
- No, and this is the part most explanations skip. Past spending can be real information — a car you've already put £2,000 of repairs into may genuinely be mechanically sound now, which is a fact about the future. The fallacy is when the size of the bill is the *only* argument for continuing.
- How do you escape a sunk cost trap?
- Ask the fresh-start question: if I were arriving today, with none of this spent, would I choose this? For a two-year side project with no users, if the honest answer is no, the two years are gone either way — and only the next two are still yours.