Bear markets in stocks are rare but have the power to spread gloom through the economy. In the last 20 years, there have been only two — one that began with the financial crisis in 2007, and the other that started with the dot-com bust in 2000. Market downturns can gather steam even without strong evidence that economic and corporate fundamentals are weakening.

“It’s kind of a feedback loop,” said Robert Shiller, a professor of economics at Yale University. “What’s happening right now, we’ve seen some declines, and that emboldens some pundit to say, ‘This is it.’ They get attention, it puts thoughts in people’s minds and they start thinking, ‘Maybe I should exit.’”

The Nasdaq is not the only group of stocks in such distress. The Russell 2000 index, which tracks shares of smaller companies, entered a bear market earlier this week. Seven of the S&P 500’s 11 industrial sectors are also at the level, led by energy stocks, which are down 28 percent from their highs earlier this year. That’s in large part because oil has been in a bear market since November.

If the broader stock market declines by more than 20 percent this year, it would end what was, by some measures, the longest bull run in history. From March 2009 until its peak in September, the S&P 500 surged 333 percent, a rally that provided a silver lining to the lackluster years that followed the financial crisis of 2008. Investors enjoyed trillions of dollars of gains from the stock market.

Those profits are not close to being wiped out. The stock market is still well above where it was even at the start of 2017. But the longer bear markets last, the more pessimism they spread.