Sunk cost fallacy

The sunk cost fallacy is when you treat time or money already spent as a reason to keep going, even when continuing is worse than stopping. Past costs that can't be recovered shouldn't by themselves decide what you do next. What matters is whether the next step is worth it from here. Treating "we've already invested so much" as a reason to invest more is a formal error in decision-making: the past expenditure doesn't logically justify future expenditure. Related: Escalation of commitment.

Examples

• We've already spent so much on this project, we can't quit now.

• I've sat through an hour of this film, so I might as well stay to the end.

• We've laid half the track, so we have to finish the line.

• I've paid for the gym for a year; I should keep going even though I hate it.