I still think the cut-over from legitimate growth to fad/fraud-driven is murky, but you're absolutely right that it's trivial to identify fad/fraud long before

I still think the cut-over from legitimate growth to fad/fraud-driven is murky, but you're absolutely right that it's trivial to identify fad/fraud long before the bust. That said, my feeling is that betting against the bubble is ultimately as bad as betting with it. Either approach can reap enormous gains, if you get the timing right. Which shakes out to: broad bets are little different from gambling. And I think investment shouldn't be gambling. As such, the 'best' (in my terms) approach to a bubble would be to hold onto your money while it grows. Then, when it bursts, pounce on the depressed prices of the good investments. It appears to me that Buffet learned that lesson after the dot-com bust and has been executing that plan on grand scale since 2008.