The Sunk Cost Fallacy: Why Smart Firms Stay Stuck
The sunk cost fallacy traps firms into chasing money already spent. Learn to spot it and reason past it in a case interview.
The sunk cost fallacy is the trap of throwing good money after bad because of money you already spent. It is the reason brilliant companies cling to dying products and capable executives defend losing strategies long past the point of sense.
Most candidates know the phrase but still let past spending sneak into a forward-looking decision. This guide fixes that. By the end you will recognize a sunk cost on sight, explain why it should never touch a future decision, and use that discipline to make a clean recommendation in a case.
The only costs that matter for a decision are the ones you can still change.
What a Sunk Cost Actually Is
Imagine you buy a nonrefundable concert ticket for $120. The night arrives and you feel sick, exhausted, and certain you will be miserable in the crowd. You go anyway, reasoning that you already paid for it. That $120 is gone whether you go or stay home. Dragging yourself out only adds a bad night to a lost $120.
The sunk cost is the $120: money already spent that no future choice can recover. It feels like it should weigh on the decision, but it cannot, because no action you take tonight brings it back. The only honest question is whether going to the concert beats staying home, starting from right now.
A sunk cost is any cost you have already paid and cannot undo. Research and development already burned, a factory already built, years already invested. These costs are real, but they belong to the past. The fallacy is letting them steer a decision that only the future can change.
Why the Past Has No Vote
The discipline is brutal but simple: when deciding what to do next, ignore every dollar you cannot get back.
Go back to the ticket. The correct decision compares only what lies ahead: a miserable night out versus a restful night in. The $120 is identical in both worlds, so it cancels out completely. Once you see that it appears on both sides of the ledger, you stop treating it as a reason for anything.
Companies break this rule constantly. A firm keeps funding a failing project because it has already sunk millions into it, as if more spending could justify the old spending. But the old millions are gone regardless. The only question that matters is whether the next dollar earns more than it could elsewhere. The past has no vote in that math.
How Consultants Use This in a Case
In a case, spotting a sunk cost is how you avoid the trap the interviewer planted on purpose.
When a case mentions all the money already invested in a plant or a product, treat that as bait. The right move is to set it aside and reason forward: what are the future costs and future returns from here? Frame your recommendation around incremental economics, the costs and benefits that still depend on the decision, never on what is already spent.
Kodak's story is the sunk cost fallacy at industrial scale. The company invented the digital camera but protected its enormous film business, unwilling to walk away from decades of investment as the future shifted under its feet. Practice this framework on a real case → Kodak 1994: The Digital Camera Dilemma on BoardroomIQ puts you in the room.
Practice this framework
Work through the Kodak 1994: The Digital Camera Dilemma case with AI coaching.
Where the Logic Gets Hard
The principle is clean, but applying it takes nerve, and interviewers push on exactly that.
Sunk costs hide behind powerful emotions: pride, fear of admitting a mistake, and loyalty to a vision. A leader who championed a project feels every dollar of it personally, which is precisely why the fallacy survives in smart organizations. Naming the emotion helps you reason past it. The discipline is not just analytical, it is the courage to abandon something you built.
Be careful not to overcorrect, though. Ignoring sunk costs does not mean ignoring everything from the past. A history of failure can still be useful evidence about future odds. The rule is narrow: do not spend more just to honor what you already spent. Use the past as data, never as debt.
How to Practice the Sunk Cost Fallacy Before Your Interviews
Audit your own bad decisions. Think of a time you finished a book you hated or kept a subscription you never used. Identify the sunk cost that held you there and rewrite the decision as if you started fresh today. This trains the forward-looking reflex.
Strip the past from a case prompt. Take any "should we continue this project" case and physically cross out every figure about money already spent. Make the call using only future costs and returns. Notice how often the answer flips once the past is gone.
Practice the abandon argument. Pick a famous company that kept investing in a doomed product. Build the case for walking away at the moment of decision, using only incremental economics. Saying it out loud builds the nerve to recommend it under pressure.
The best way to practice the sunk cost fallacy is under realistic pressure, with a case that fights back.