What it means
Good decisions compare what happens from this point forward under each option. That means looking only at costs and benefits that are still avoidable or still to come, which economists call incremental or relevant costs.
Anything already paid and unrecoverable is the same under every option, so including it changes nothing except your emotional attachment to one answer. The fallacy is the tendency to keep investing because of what has already gone in, described informally as throwing good money after bad.
It shows up in failing software projects, marketing campaigns that are not converting, product lines nobody wants and, at a personal level, in seats at films people stopped enjoying twenty minutes ago. The practical test is a simple question: if I were starting today with none of this money spent, would I choose to invest the remaining amount for the expected return?
If the honest answer is no, the past spending does not change it. If the answer is yes, then continuing is right regardless of how much has been wasted getting here.
Not everything already spent is genuinely sunk, which is where care is needed. If equipment can be sold, a contract cancelled for a partial refund or staff redeployed to other work, that recoverable amount is a live consideration and belongs in the decision.
Sunk cost thinking also interacts with accounting. A large capitalised asset still sits on the balance sheet, and writing it off feels like creating a loss, but the loss occurred when the money was spent, and the write-off merely recognises it.
Managers who delay a rational shutdown to avoid an impairment charge are letting reporting optics drive an economic decision.
In practice
Real-world examples.
Example
A software firm has spent eighteen months and $1,200,000 building an internal system that is still not usable. When a proven third-party product is found for $90,000 a year, the correct comparison is the cost of finishing against the cost of buying, with the $1,200,000 excluded entirely.
Example
A restaurant group spent $180,000 fitting out a site that is not attracting covers. It negotiates a lease surrender for $40,000, and because that fee is a future avoidable cost while the fit-out is not, the decision turns on whether forecast future losses exceed $40,000.
Example
A manufacturer bought $220,000 of specialised tooling for a product line that is now loss-making. The tooling can be sold for $60,000, so the $60,000 is relevant to the decision to close the line while the remaining $160,000 of original cost is sunk.
Formula
Calculation
Decision rule: Continue if (Expected future benefits - Expected future costs) is greater than the net benefit of stopping. Sunk costs are excluded from both sides.
A consumer goods company has spent $400,000 developing a new blender. The work is 70% complete, the spending is unrecoverable, and finishing the product will cost a further $250,000. Marketing forecasts lifetime contribution of $300,000 from the launch.
Forward-looking comparison, ignoring the $400,000:
Continue: $300,000 - $250,000 = $50,000 net benefit
Stop: $0
Continuing is the better decision by $50,000, even though the project loses money overall. Confirming that with total figures: continuing gives -$400,000 - $250,000 + $300,000 = -$350,000, while stopping gives -$400,000, and -$350,000 is $50,000 better than -$400,000.
Now suppose a competitor launches first and the contribution forecast falls to $180,000. Continuing gives $180,000 - $250,000 = -$70,000, so stopping is now better by $70,000. The $400,000 already spent is identical in both scenarios and never affects which way the decision goes.Case study
Seen in the real world.
Peregrine Devices is an illustrative and entirely fictional consumer products company used to show the sunk cost fallacy in ordinary corporate form. It had spent $400,000 developing a premium blender when a rival launched a near-identical product at a lower price, and the forward contribution forecast dropped to $180,000 against a remaining build cost of $250,000.
The product director argued that stopping would waste $400,000 of good work, and that argument won two consecutive review meetings. Finance eventually reframed the decision on a single slide showing only the numbers that were still in play, namely $180,000 of expected contribution against $250,000 of remaining spend, a forward loss of $70,000, and the project was cancelled that afternoon.
The illustrative point is that the $400,000 was lost before the meeting started and no decision made in the room could bring it back. Peregrine went further and changed its stage-gate template so that every review paper reports remaining cost and remaining benefit only, with cumulative spend shown separately as historical information rather than as a factor in the recommendation.
Watch out
Common mistakes.
- Justifying further investment on the grounds that stopping would waste what has already been spent, when that money is equally gone under both options.
- Treating every past cost as sunk without checking recoverability, when assets that can be resold or contracts that can be cancelled produce cash that genuinely belongs in the decision.
- Delaying a rational shutdown to avoid recognising an impairment in the accounts, which turns a reporting preference into an ongoing cash loss.
Questions
People also ask.
Is a sunk cost the same as a fixed cost?
No, because a fixed cost does not vary with output but may still be avoidable in future by closing a site or ending a lease, whereas a sunk cost has already been incurred and cannot be recovered at all.
Should sunk costs ever appear in a business case?
They can appear as historical context for accountability and lessons learned, but they must be excluded from the numbers that drive the recommendation.
Why is the fallacy so persistent?
Because people dislike admitting a loss and often feel personally accountable for the original decision, so continuing preserves the hope that the spending will eventually be justified.
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