The Stock Market Is Doing Something It's Only Done 1 Time Since 1871. Should You Be Worried for 2026?

The S&P 500 (SNPINDEX: ^GSPC) index surged past 7,000 for the first time in late January, driven by rising investor optimism about artificial intelligence (AI) and robust corporate earnings. However, over the next few days, mixed earnings performance from large technology companies dramatically cooled investor sentiment.

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Wall Street is also flashing a warning that investors may find difficult to ignore. The cyclically adjusted price-to-earnings (CAPE) ratio, also known as the Shiller P/E ratio, has climbed above 40 for only the second time since 1871.

Shiller CAPE ratio

The Shiller CAPE ratio measures the current price of the S&P 500 index relative to the 10-year moving average of inflation-adjusted earnings. By smoothing out short-term earnings fluctuations, it provides a more stable view of market valuation than the standard price-to-earnings ratio.

The Shiller CAPE ratio for the S&P 500 at the time of this writing is roughly 40.5, more than double the historical long-term average of about 16 to 18. When the ratio rises far above the historical norm, it signals that investors are paying unusually high prices for each dollar of earnings. This condition has historically been difficult to sustain over the long term.

The only other time when Shiller CAPE exceeded 40 was in December 1999, during the dot-com bubble. That period was followed by a "lost decade for stocks," from December 1999 to December 2009, when the S&P 500 delivered an annualized total return of negative 0.9%.

Expectations for 2026

The CAPE ratio is not a timing tool. Hence, elevated valuations can persist longer than expected. In December 1999, although the CAPE ratio crossed 40, stocks continued to climb for the next three months.

Currently, the implied future annual return for the S&P 500 is just 1.5%, according to GuruFocus. While estimates like these are inherently uncertain, history shows that extremely high CAPE readings have typically been associated with lower returns in the next 10 to 20 years.

Extreme valuations eventually correct, either through falling prices, slower earnings growth, or long periods of sideways performance. Whether that adjustment begins in 2026 or later, today's stretched CAPE ratio appears to be a warning for investors to temper long-term return expectations.

Manali Pradhan, CFA has no position in any of the stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.