Sunk Cost Fallacy
Continuing with something because of money, time, or effort already spent — even when the rational move is to cut your losses.
Origin
Economists draw a sharp line between sunk costs (already spent, gone) and marginal costs (what it will cost to continue). The sunk-cost fallacy is treating the past spend as a reason to keep going. The Concorde project is the canonical large-scale example: both governments knew the plane would lose money, but both had already spent too much to stop. Behavioral economists Daniel Kahneman and Amos Tversky tied the fallacy to loss aversion.
Modern usage
Massively useful piece of vocabulary, applied to relationships ('we've been together for ten years, so…'), bad jobs, half-finished books, half-finished degrees, half-finished startups. 'Don't fall for the sunk-cost fallacy' is standard advice for breakups and pivots. The investing-and-folding sense crossed into ordinary conversation through poker writing and self-help.
In the wild
I've already watched four hours of this show — I have to finish. (Pure sunk cost.)— common usage
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