An asset bubble is easy to identify afterwards and notoriously hard to call at the time. The difficulty is not that warning signs are absent but that the same signs appear during justified revaluations: prices rise sharply when a technology genuinely changes what a company is worth, and rise sharply when it does not. Economists who insisted through the late 1990s that internet valuations were unsupportable were vindicated in 2001, and would have been wrong had they said the same of the personal computer in 1985. Because the cost of a false alarm falls on the person who raises it, while the cost of silence falls on everybody, the incentives facing anyone in a position to warn are poorly arranged.