The U.S. stock market has delivered decent returns in 2026 despite persistent economic uncertainty created by tariffs and, more recently, elevated oil prices tied to the Iran war. This year, the S&P 500 (SNPINDEX: ^GSPC) has added 8% and the Nasdaq Composite (NASDAQINDEX: ^IXIC) has added 7%.
However, the S&P 500 recently flashed a warning last seen during the dot-com era, and it hints at big losses in the stock market over the next three years. Read on to learn more.
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The S&P 500 flashes a warning seen only once before
In 1988, economist Robert Shiller introduced the cyclically adjusted price-to-earnings (CAPE) ratio as means of evaluating entire stock market indexes. Whereas the traditional price-to-earnings ratio can be distorted by cyclical fluctuations in earnings, the CAPE ratio eliminates that noise by averaging inflation-adjusted earnings from the past decade.
The S&P 500 recorded an average CAPE ratio of 40.9 in June, the second straight monthly reading above 40. Not only is that well above the 20-year average of 27.6, but the last two months mark the first time since the dot-com bubble (in the late 1990s and early 2000s) that the S&P 500 recorded a CAPE ratio above 40.
Unfortunately, the index’s rich valuation hints at substantial downside in the stock market. The following chart shows the S&P 500’s best, worst, and average returns over different periods following a monthly CAPE ratio above 40.
Data source: Robert Shiller, YCharts.
There are two important data points in the chart. First, the S&P 500 has never generated a positive three-year return following a monthly CAPE ratio above 40. Second, if the S&P 500’s future returns match the historical average, the index will drop 30% by July 2029.
Of course, past performance does not guarantee future results. The CAPE ratio did predict the dot-com crash, but the internet boom was different than the artificial intelligence (AI) boom. The internet did not reach mainstream adoption for more than a decade, but AI has achieved mainstream adoption in under five years.
In fact, AI ranks among the “fastest-adopted technologies in history, with nearly one in four American firms deploying it at scale,” according to Justin Bieman, global investment strategist at JPMorgan Chase. That means AI could become a material source of corporate profits more quickly than the internet.
