Animal spirits—the gut feeling that, yes, this is the time to buy a house or that sleeper stock—drive people to overconfidence and rash decision making during a boom. These feelings can quickly transmute into panic as anxiety rises and the market heads in the other direction.

Animal spirits propelled Internet stocks to indefensible heights during the dot-com boom and drove their values earthward just a few years later. They were present again when reckless lenders took advantage of low-interest rates to proffer adjustable-rate mortgages on risky, subprime borrowers. A phenomenon like money illusion prevailed: the borrowers of these mortgages failed to calculate what would happen if interest rates rose, which is exactly what happened during the middle of the decade, causing massive numbers of foreclosures and defaults.