What is the sunk cost fallacy, and why is "I've already invested too much to stop" the wrong way to decide?

Continuing something — a project, a relationship, a film — only because of the time, money or effort already spent, even when stopping is the better choice going forward.

The mechanism is loss aversion plus a desire to feel consistent: quitting forces us to admit the past investment is gone and was "wasted", which feels like a loss, so we keep pouring in resources to avoid that feeling. But money and time already spent are sunk — they're gone whether you continue or not. The only rational question is whether future benefit outweighs future cost.

Example: You're an hour into a boring three-hour film. Staying because "I've already sat through an hour" wastes two more hours; the lost hour is gone either way. A rational chooser decides purely on whether the next two hours are worth it.

Tip: Ask: "Knowing only what I know now, would I start this today?" If no, the past spending shouldn't keep you in it.