Let's say you buy a $10 movie ticket.
But 15 minutes in, you realise this movie isn't what you expected it to be.
However, because you feel like you're supposed to get your money's worth, you decide to watch the rest of the 2 hour movie.

This decision is based on your loss of $10.
But it isn't an optimal one.
A more logical thing to do, would have been to get up and leave after you had realised this movie wasn't what you wanted to watch.

Wouldn't you be better off, had you only wasted $10 and 15 minutes of your time, instead of wasting $10 and 2 hours?
Had you left early, you would've essentially gained 2 hours of extra time.
Those $10 you spent were gone or "sunk" either way. You couldn't get them back, whether you watched the rest of the movie or not.

And that phenomenon is what psychologists and economists call: the sunk cost fallacy.
It's one of the more common cognitive biases that clouds our judgement.
Instead of making a decision based on potential future returns, we make them on our past losses.

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