One silver lining of the stock market’s fluctuations in recent months: Individual investors say they have learned a thing or two about investing along the way. The S&P 500 has been stuck in a bear market—down more than 20% from its high—for 221 days. That is the longest such stretch since 1973, surpassing the selloffs that coincided with the bursting of the dot-com bubble in 2001 and the financial crisis of 2008.

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