Sunk Cost: Water You've Already Spilled
A sunk cost is money or time already spent that you can never get back. When you decide rationally, it shouldn't affect what you do next.
Example: you bought a movie ticket, watched twenty minutes, and realized it's terrible. Leave or sit through it?
The ticket money is already spent; sitting through it just throws in another hour and a half—the money won't come back because you stuck it out. The real question is: "From now on, which is better for me, finishing or leaving?"
Why We Can Never Let Go
Even knowing the spilled water can't be recovered, people still think, "I've put in so much already—giving up would be such a waste." So they sink in deeper and deeper—this is the sunk cost fallacy.
You've queued two hours and you're almost there, then you hear this place is actually mediocre, but you think "I've waited this long, I can't leave now." In fact the time you spend from here on has nothing to do with "I already waited two hours"—only with "is this meal worth waiting for any longer."
Survivorship Bias: Seeing Only the Ones Who Made It
Survivorship bias: we only get to see the cases that "survived and succeeded," and can't see the ones that quietly failed and vanished—so we draw the wrong conclusion.
Example: "All those big bosses never finished college, so school is useless."
You only see the few who dropped out and succeeded, not the far larger number who dropped out and quietly suffered for it—they never make the news. Counting only the winners badly overestimates how often a path works out.
(The classic example is WWII statistician Abraham Wald analyzing the bullet holes in returning warplanes: the parts to reinforce were precisely the ones with fewer holes, because the planes hit there never made it back.)
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You've watched twenty minutes of a movie and it's genuinely terrible. What should your rational standard for deciding be?
答案：Look only at whether staying or leaving from here is better; the ticket money is spent and no longer counts
The ticket money and the twenty minutes already spent are sunk costs that can't be recovered. A rational decision looks only at which choice is better from now forward. The second and third fall into the sunk cost fallacy.
"Successful entrepreneurs all love taking risks, so to succeed you should take more risks." What's the main problem with this inference?
答案：Survivorship bias: it looks only at the successful and ignores those who took risks, failed, and vanished
Counting only the "surviving successes" and ignoring the many who took the same risks and failed overestimates the upside of risk-taking—survivorship bias.
True or false: thoughts like "I'm already here" and "the money's spent, I can't waste it" are often the sunk cost fallacy at work.
答案：True
Exactly. These lines treat unrecoverable spending as a reason to keep investing—the most typical expression of the sunk cost fallacy.
True or false: counting only the cases that "survived and succeeded" while ignoring the silent failures will make us overestimate how often a path works out.
答案：True
This is exactly the consequence of survivorship bias: failures go unseen while winners are mentioned over and over, so the success rate gets badly overestimated.
A cost that's already been spent, can never be recovered, and shouldn't affect later decisions is called a ____ cost.
答案：sunk
This kind of unrecoverable cost is called a sunk cost; a rational decision should leave it out and look only at gains and losses from now forward.
To find out "whether a certain exam-prep program is really useful," which approach best avoids survivorship bias?
答案：Also seek out people who took the program but didn't pass, rather than only hearing from those who got in
Including the "didn't succeed" cases is what avoids the survivorship bias of looking only at winners. The second looks only at survivors; the third is sunk cost; the fourth is confirmation bias.