Warren Buffett's Favorite Stock Market Indicator Is Flashing Red. Here's What History Warns Is Coming Next.

It's no secret the S&P 500 (SNPINDEX: ^GSPC) has enjoyed an unprecedented rally in recent years, fueled mostly by enthusiasm surrounding artificial intelligence (AI) and investments in technology infrastructure. Ironically enough, it's times like these that usually have contrarian investors like Warren Buffett sitting on the sidelines instead of following the crowd.

Buffett's favorite valuation tool suggests his concerns may be justified. With stocks now trading well above levels Buffett views as extreme, history offers some important clues about what could happen next.

The Buffett indicator is flashing red

In an article published by Fortune in December 2001, Buffett explained how investors could assess whether the overall stock market was reasonably valued. He described the relationship between total U.S. stock market capitalization and the country's gross national product (GNP) as "probably the best single measure of where valuations stand at any given moment."

Imagine the combined market capitalization of every publicly traded American was $200 trillion while the U.S. economy produced $100 trillion in annual output. Dividing those figures generates a Buffett indicator of 200%. In other words, investors are valuing the stock market at twice the country's annual economic output.

There is one important distinction to note when calculating the Buffett indicator. Buffett originally referred to gross national product (GNP), which measures economic output attributable to U.S. residents including income earned through overseas activity. However, a more common way of calculating the Buffett indicator substitutes GNP with gross domestic product (GDP). GDP measures economic activity occurring within U.S. borders, regardless of who produces it.

According to GuruFocus, the current U.S. market-cap-to-GDP ratio hovers around 233%. Buffett says that when this indicator approaches 200% investors are "playing with fire." While the current excess in the market doesn't guarantee a crash is imminent, it does suggest investors are paying historically high prices for stocks.

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History offers a stark warning when valuations become expensive

Two periods are particularly relevant today's bull market: the dot-com bubble and the market downturn between 2021 and 2022. During the late 1990s, investors were captivated by the internet. Although they were correct about the internet's revolutionary potential, many technology businesses at the time commanded unusually high valuations that proved unsustainable given the lack of revenue and profit from these companies.

Prior to the dot-com bubble bursting, the Buffett indicator peaked around 143%. Between March 2000 and October 2002, the S&P 500 declined approximately 49% while the Nasdaq Composite (NASDAQINDEX: ^IXIC) plunged nearly 78%. What's even more devastating is that the S&P 500 did not recover its pre-dot-com high until June 2007.

Something similar happened nearly two decades later. In December 2021, the Buffett indicator was sitting just about Buffett's 200% warning sign. This was reflective of elevated stock valuations during the pandemic-era rally.

However, by mid-2022 inflation had started to surge, causing the Federal Reserve to aggressively raise interest rates. Naturally, investors began reassessing growth stocks. Between January and October 2022, the S&P 500 declined as much as 25%.

These episodes illustrate how stretched valuations often precede meaningful corrections in the market. In addition, the timing and severity of those declines are impossible to predict using the Buffett indicator in isolation.

Is a stock market crash coming?

The Buffett indicator's current reading deserves attention, but I wouldn't interpret it as absolute proof that a crash is coming.

Today's market is considerably different from the dot-com era. AI leaders such as Nvidia, Microsoft, Alphabet, Amazon, and Meta Platforms generate substantial profits because their businesses are supported by durable revenue streams rather than speculative ideas.

Meanwhile, many American companies now generate significant revenue internationally. Since the Buffett indicator compares worldwide market capitalizations with U.S. GDP, globalization can make the ratio appear higher relative to prior decades.

Nevertheless, overvalued stocks will always be vulnerable when expectations become unrealistic. If AI infrastructure spending slows, interest rates continue rising, or earnings miss expectations, investors will likely reassess the premiums they're currently paying.

This is why the smartest approach is to focus on high-quality companies with competitive advantages, strong balance sheets, consistent cash flow, and diversified business models.

Ultimately, the Buffett indicator isn't a crystal ball. But history suggests that buying stocks at historically expensive valuations will lead to disappointing returns and prolonged periods of volatility. Nevertheless, smart investors will remain invested because patience and discipline will help position your portfolio for long-term success no matter what comes next.

Adam Spatacco has positions in Alphabet, Amazon, Microsoft, and Nvidia. The Motley Fool has positions in and recommends Alphabet, Amazon, Meta Platforms, Microsoft, and Nvidia. The Motley Fool has a disclosure policy.