Many bullish stock market participants shrug off worries of a stock-market bubble, saying the market's valuations aren't nearly as stretched in 1999-2000.
That may be true, says Charlie Bilello director of research for Pension Partners, a money manager.
"The issue, though, is that every bull market doesn’t go through a 2000-like bubble before it becomes a problem," he writes on Yahoo.
"After all, 2000 was the peak of the greatest U.S. stock market bubble we have ever seen."
Robert Shiller's cyclically-adjusted price-earnings ratio, which includes 10 years of earnings, for the S&P 500 reached almost 45 in 1999, compared to 26 now.
But the S&P 500 has suffered 20 bear markets since 1929, and only one hit after "our collective definition of a bubble," Bilello writes.
"Therefore, to argue that stocks are a good buy today and can’t go down because they have not yet reached the extremes of the greatest bubble in history" is simply wrong, he says.
Jeremy Siegel, a finance professor at University of Pennsylvania, doesn't share Bilello's concern.
He predicts the Dow Jones Industrial Average will hit 18,000 by year-end, maybe even 19,000.
The Dow stood at 17,040 Friday morning.
Low interest rates and inflation will combine with strong earnings to boost stocks, he told CNBC.
As for potential pitfalls, "we live in a world of uncertainty, and bull markets climb the wall of worry," Siegel said. "When we see nothing in the future that can worry us at all, I'll get worried, and I'll probably tell people to sell stocks."
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