Sunk Cost Fallacy Explained (And When Finishing Is Right)

A sunk cost is a cost that has already been incurred and cannot be recovered no matter what you decide next. The sunk cost fallacy is letting that unrecoverable spending influence a decision it cannot logically affect, usually by continuing something purely because of what has already been put into it. The correct rule is short: only costs and benefits that change with the decision belong in the decision.

The idea belongs to economics but is taught just as often in psychology, business strategy, and project management, because it explains why companies keep funding failing projects and why people sit through films they are not enjoying. This guide gives the decision rule, works two examples, and covers the case people usually get backwards.

The rule

When choosing between continuing and stopping, compare only what differs between the two paths. Money already spent is identical under both, so it cancels out and cannot change which one is better. What matters is the additional cost of continuing against the additional benefit of continuing.

This is marginal analysis applied to time. The question is never what have I put into this, it is what happens from here.

A cost is sunk when it cannot be recovered. If you can resell the equipment, its resale value is not sunk. If the deposit is refundable, it is not sunk. The test is whether the money comes back under any available option, not whether it was spent in the past.

The concert ticket

You paid $80 for a ticket to a concert tonight. It cannot be resold or refunded. On the night, a storm arrives and you no longer want to go, and you would rather stay home.

The $80 is gone under both options. Going does not recover it and staying home does not lose it a second time. So the only question is whether an evening at the concert in a storm beats an evening at home. If it does not, stay home.

The familiar objection, that staying home wastes the $80, has the accounting wrong. The $80 was spent when the ticket was bought. Going out in a storm you would rather avoid adds a bad evening to a payment you cannot undo, which makes the outcome worse rather than better.

When the fallacy tells you to quit and the answer is finish

This is the case most people get backwards, and it is worth working through carefully because it shows the rule is not just a fancy way of saying give up.

A firm has spent $2 million developing a product. Completing it requires a further $1 million. The finished product is expected to earn $1.5 million in revenue.

The tempting reasoning: total cost is $3 million against $1.5 million of revenue, a loss of $1.5 million, so abandon it. That reasoning uses the sunk $2 million, and it is wrong.

Finishing brings in $1.5 million for an extra $1 million, which is $500,000 better than abandoning. The project is a failure overall, and someone should probably answer for the original decision, but abandoning now would waste $500,000 on top of the loss already taken.

So the sunk cost principle cuts both ways. It says stop honoring past spending when continuing is bad, and it equally says keep going when the remaining economics work, however painful the history.

Sunk cost and opportunity cost

These two are often taught side by side because they are opposites in a useful way.

A decision is made correctly when sunk costs are excluded and opportunity costs are included. Most poor decisions manage to do exactly the reverse, counting the money already spent while ignoring the alternatives being given up.

The same distinction explains why accountants and economists report different profit figures, which is covered in economic profit vs accounting profit.

Why people fall for it

The fallacy is persistent because the pull behind it is real. Abandoning a project means admitting the original spending achieved nothing, and people would rather avoid that feeling than lose more money. Loss aversion makes the already-taken loss feel avoidable as long as the project is still alive.

Organizations make it worse. The person who approved the spending is often the person deciding whether to continue, and continuing defers the reckoning. This is why large projects are frequently reviewed by someone who did not authorize them, and why the phrase throwing good money after bad exists.

Where it shows up

Business. Firms continue failing product lines, acquisitions, and infrastructure projects because of what has been committed. The Concorde airliner is the standard case, so much so that the sunk cost fallacy is sometimes called the Concorde fallacy.

Firms deciding whether to shut down. In the short run a firm should keep operating as long as revenue covers its variable costs, because fixed costs are being paid either way. Applying the sunk cost rule correctly here is a standard exam question and a real management decision.

Personal choices. Finishing a book you dislike, staying in a course because of the terms already completed, or holding a losing investment because of what was paid for it are all the same error.

Common mistakes

Treating every past cost as sunk. Only unrecoverable ones qualify. If equipment can be sold for $30,000, that $30,000 is very much part of the decision, because it is available under one option and not the other.

Assuming the rule always means quit. As the $2 million example shows, correctly ignoring sunk costs often means finishing something that looks like a disaster on paper.

Confusing it with fixed cost. Fixed costs do not vary with output but may still be avoidable by shutting down, in which case they are not sunk. A lease you can exit is fixed but recoverable.

Forgetting the decision is always forward-looking. If you find yourself justifying a choice with how much has already gone in, you have left economics and entered bookkeeping about feelings.

Practice and connect

Sunk cost is one half of the pair of ideas that make marginal analysis work, the other being opportunity cost. Make sure you can define a sunk cost in one sentence, run the concert and project examples in both directions, and explain why a firm keeps operating when revenue covers variable cost. Reinforce the terms in the sunk cost glossary entry, then apply the reasoning across the basic concepts module.