What it means
A sunk cost is any outlay that cannot be reversed, whether it is a software build, a marketing campaign, a lease premium or years of management attention. Economics treats such costs as irrelevant to decisions, because no choice on the table can bring the money back.
Human beings, unfortunately, treat them as very relevant indeed. The pull comes from a dislike of admitting waste, reinforced by the fact that the person deciding is usually the person who approved the original spend.
Abandoning a project feels like declaring the earlier money wasted, even though it was wasted at the moment the project stopped being worthwhile, not at the moment somebody said so out loud. In business the fallacy shows up as failing systems that keep getting extended, product lines that are never retired and acquisitions defended long after the case for them collapsed.
It also drives escalation of commitment, where each new injection of cash makes the next one psychologically easier to approve. The cost compounds quietly, because capital tied up in a failing project is capital unavailable to a good one.
The remedy is procedural rather than motivational. Stage gates with abandonment criteria agreed in advance, review panels that exclude whoever authorised the original budget, and appraisal that starts from today's position all reduce the pull.
Framing the choice as a completely new investment decision is the simplest discipline of all. One caution is worth stating, because not every continuation is a fallacy.
Finishing a project can be entirely correct when the remaining benefits genuinely exceed the remaining costs, and the fallacy only bites when past spending, rather than that forward comparison, is what drives the answer.
In practice
Real-world examples.
Example
A retailer has spent $1,400,000 on a new till system that still fails at peak times. Rather than counting the money spent, the board compares $500,000 of remaining development cost with the $300,000 of annual savings the system would deliver, and approves completion on that basis alone.
Example
A biotechnology firm halts a compound after phase two results disappoint, despite six years and $40,000,000 of research behind it. The chief executive tells investors the only relevant question was whether the next $60,000,000 was the best available use of that money.
Example
A marketing director keeps funding a trade magazine campaign because the annual contract was signed and paid up front. The contract is a sunk cost, and the honest comparison is between the cost of producing new adverts and the leads they are likely to generate.
Think of it
“Sunk cost fallacy is letting past spending affect future decisions-those costs are gone.
Formula
Calculation
Continue only if: Expected future benefits > Expected future costs, with all past spending excluded from both sides of the comparison
Larkfield Software has already spent $2,000,000 building a customer portal. Completing it will cost a further $800,000, and the sales team now expects it to generate $600,000 of value over its useful life.
The forward looking test compares $600,000 of benefit with $800,000 of remaining cost, giving $600,000 - $800,000 = -$200,000, so the project should be stopped. The sunk cost view instead adds the money already gone, comparing $2,000,000 + $800,000 = $2,800,000 of total spend with $600,000 of benefit and concluding the team must press on so the $2,000,000 is not wasted. Following that logic turns a $2,000,000 loss into a $2,000,000 + $800,000 - $600,000 = $2,200,000 loss.Case study
Seen in the real world.
This is an illustrative and fictional example. Verity Rail Systems, an invented signalling contractor, spent three years and $6,000,000 developing a maintenance platform intended for sale to regional operators. By the third year only two operators had shown interest, and both wanted features that would add another $2,500,000 of development.
Every review meeting reached the same conclusion, that stopping now would waste $6,000,000. The finance director eventually rewrote the paper so the sunk figure appeared only as a footnote, and the front page compared $2,500,000 of future cost against a realistic $900,000 of expected revenue.
Presented that way, the fictional board took eleven minutes to close the project. The team was moved onto a services contract that generated $3,200,000 over the following two years, which is the money the sunk cost fallacy had been quietly costing them all along.
Watch out
Common mistakes.
- Including money already spent in a decision about whether to continue, which reliably pushes the answer towards throwing good money after bad.
- Confusing a sunk cost with a fixed cost, when a fixed cost may still be avoidable in future while a sunk cost never is.
- Asking the person who championed the original investment to decide alone whether it should continue, which builds the bias straight into the process.
Questions
People also ask.
Is every decision to finish a struggling project a sunk cost fallacy?
No, continuing is correct whenever remaining benefits exceed remaining costs, and the fallacy applies only when past spend is doing the persuading.
How does the sunk cost fallacy differ from opportunity cost?
Sunk cost is money already gone and irrelevant, while opportunity cost is the value of the best alternative you give up and is highly relevant.
What is the simplest way to avoid it in practice?
Ask what you would do if you were arriving at the decision today with no history, and require the answer in writing before any further funding is released.
From the founder's library
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