Don’t worry about how sky-high stock market prices are today.  Or Bitcoin – its price is now $59,000 (having appreciated 500% over the past 12 months).  Or the price of GameStop (from $4 to $483 in 6 months) or of sports trading cards ($1.3 million...

The obverse of “Rational Exuberance” is “Irrational Exuberance,” a phrase popularized in December 1996 by then-Federal Reserve Chairman Alan Greenspan when he warned investors that unjustified internet optimism had crept into stock market valuations.  As a consequence, he warned, soaring valuation assumptions based on rosy...