The S&P 500 (^GSPC -1.09%) has been humming along in 2026, up 14% as of Oct. 13. Still, investors may feel like they have to keep looking over their shoulders, as inflation remains stubborn, U.S. families falling behind on loan obligations are at their highest levels since 2010, and U.S. debt levels sit above $40 trillion.

That's why, despite this year's rally, some may be worried about a pullback deep enough to trigger a bear market, defined as a drop of 20% or more from a recent high. There's no way to outrun a bear market; if one is going to happen, it will.

Still, successful investors know the playbook that helps them avoid costly mistakes when stocks seem to be doing nothing but heading lower.

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The cost of selling during a bear market

When stock prices are dropping, it's easy to feel that selling is better than doing nothing. After all, it only feels natural to protect what you have left rather than watch the value of your investment decrease even further.

What history suggests, however, is that panic selling can be a costly mistake.

It sounds counterintuitive to a natural perception of how stocks perform, but some of the best days for the S&P 500 came during a bear market. From 1996 through 2025, 48% of the best days for the S&P 500 happened during a bear market, according to Hartford Funds. That was followed by 28% of the best days occurring during the first two months of a bull market, and 24% during the rest of the bull market. And the cost of missing those best days is jarring.

A $10,000 investment in the S&P 500 in 1996 would have been worth $192,167 by the end of 2025. Missing just the 10 best days, however, would have lowered that investment by 56% to $85,490. It gets even worse from there, as missing the 20 best days would have yielded a $49,551 return, and missing the 30 best days would have turned that $10,000 investment into just $31,123.

The investing game plan

Wanting to sell during a market downturn is natural. The good news is that the selling itch doesn't have to make the decisions.

Instead, a portfolio already filled with high-conviction stocks can help mitigate the risk of knee-jerk reactions. A portfolio filled with mostly speculative positions can be hit hard when the market faces a downturn and is more likely to cause panic selling. Successful investors fill their portfolios with stocks that they are comfortable buying more of if prices drop.

Which stocks those are will vary depending on each person's goals and risk tolerance. However, Warren Buffett offered some helpful advice in a 1996 Berkshire Hathaway shareholder letter that remains relevant today. "If you aren't willing to own a stock for ten years, don't even think about owning it for ten minutes."