gwern comments on Markets are Anti-Inductive - Less Wrong
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The way I think of it: imagine you're in the middle of Tulipomania and you're thinking about sinking your entire fortune into a tulip bulb. You reason that the bulb is largely worthless since its function is to look nice and the value only comes from your expectation that you will sell it to someone else for more, who will reason the same way. This whole spiral feeds on something: credit, suckers, etc. The rational thing is to jump off just before the spiral runs out (and being an exponential process, the spiral won't starve slowly but will crash hard - it won't be that the banks are demanding somewhat higher interest rates for tulip-backed loans, but that there won't be any at all). Of course, the other people are reasoning the same way, so they will be looking for the jump-off too; since one of the things the spiral is feeding on is you, your jump will contribute to the crash.
But what sparks you to think that the crash is about to occur (and cause a self-fulfilling prophecy)? Could be some exogenous event like a war or terrorist attack, but there must be one sooner or later: there are only so many tulips to buy & sell and investors are only willing to hold tulips for so long, they want to cash out at some point, but for there to be a seller, there must be a buyer...
If you can calculate this limit, then you know when you must jump off before, and so does everyone else who isn't a fool, and you become a pack of penguins inching nervously to the edge, wondering who the first one shoved over will be to test the water for killer whales. At that point, a crash is only logical. The upper limit for bubbles is where not enough people can afford to buy the good or to hold the good; the lower limit is where the good is unfairly priced in the sense that it can be diverted to some other use (eg. we can imagine the US dollar depreciating only so far, because if a dollar is cheap enough, we could start buying it in bulk, shredding it, and turning it into fertilizer or linen or something).
I don't know if this helps you, but it's the way I see it.