More important was the price that investors were willing to pay for a dollar of earnings. That went from a multiple of 13 times to 23.6 times over the decade ended Dec. 31, 2021. A multiple of 15 to 16 is about the historical average.

Guessing what prices investors will pay in the future, and when or whether they will revert to the mean, is notoriously hard. The recent selloff could be the early stages of that adjustment, though, according to Christopher Bloomstran, a value-investing veteran who is president of Semper Augustus. He wrote in an email interview that tightening monetary policy is likely to be the catalyst.

“The Fed has a perfect record popping bubbles. They aren’t likely to fail this time,” Mr. Bloomstran wrote.

Another prominent value investor, Jeremy Grantham, co-founder of the asset manager GMO, wrote in January that U.S. stocks had entered their fourth “superbubble” of the past 100 years and that he expected them to drop by half. In addition to quantitative reasons such as statistical deviation from long-term trends, he cited a more subjective historical cue akin to ringing a bell near the top—“crazy” speculation, this time in meme stocks, EV makers, cryptocurrencies and NFTs.