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Sunk Cost Fallacy

Why It Matters

Sunk cost reasoning is everywhere and costly. Organizations throw good money after bad. Individuals stay in bad relationships, careers, and projects because of prior investment. Recognizing sunk costs as irrelevant is difficult but essential.

The Key Move

For any continuing commitment, ask: would I make this choice if I hadn't already invested? Ignore prior investment; evaluate only future costs and benefits. If you wouldn't start this project today, you should quit it.

Classic Application

Concorde fallacy. Britain and France continued funding the Concorde supersonic jet long after it was clear the project was economically unviable - because they'd already invested so much. The prior investment was a reason to continue, not to stop.

Surprising Application

Relationship persistence. People stay in bad relationships partly because they've invested years. "I've already given this so much" becomes a reason to give more, even when the relationship has no future. The sunk cost keeps them stuck.

Failure Modes

  • Treating all persistence as sunk cost fallacy - sometimes continuation is genuinely optimal
  • Ignoring that quitting has costs (reputation, switching costs) beyond the psychology
  • Missing that sunk cost reasoning can serve as a commitment device - it's not always irrational
  • Over-quitting because you've learned about sunk costs - don't abandon good projects
  • Ignoring emotional and identity investments that make "sunk costs" more complex

Go Deeper

Thaler's early work on mental accounting and sunk costs. Arkes and Blumer on sunk cost experiments.