In the landscape of investment, stocks have consistently outperformed other asset classes, such as bonds, commodities, and real estate, over the last century. The Dow Jones Industrial Average, S&P 500, and Nasdaq Composite have all shown remarkable annualized returns that are difficult for other asset classes to rival. However, potential investors should be mindful that the stock market is not without its periods of volatility and decline.
Various catalysts can lead to market downturns, and although many concerns may diminish over time, some evolve into significant challenges for investors. One notable issue raised recently by Federal Reserve Chair Jerome Powell highlights a classic risk associated with stock market valuations.
Typically, members of the Federal Open Market Committee (FOMC), which sets U.S. monetary policy, refrains from discussing stock market dynamics directly. Their primary focus is to uphold the dual mandate of maximizing employment and stabilizing prices, with the stock performance of indices like the Dow, S&P 500, and Nasdaq generally not impacting their policy decisions. However, there are instances where Fed chairs comment directly on market valuations.
Alan Greenspan notably warned of “irrational exuberance” in 1996, a phrase that underscored the rapid rise of stock prices fueled by the internet boom. Despite his remarks, it wasn’t until three years later that the actual market correction began. More recently, in September 2025, Powell broke away from tradition and highlighted the issue of equity valuations, acknowledging that “by many measures, for example, equity prices are fairly highly valued.”
This statement has significant implications for Wall Street. Following Powell’s remarks, the broader market continued to climb, with indices reaching psychologically significant milestones. However, Powell’s observations about overvalued equity prices echo historical trends. Investors tend to look at long-term patterns, and some valuation metrics—especially the S&P 500’s Shiller Price-to-Earnings (P/E) Ratio, also known as the Cyclically Adjusted P/E Ratio—are raising alarms.
While conventional P/E ratios may fluctuate due to short-term macroeconomic shocks, the Shiller P/E offers a broad perspective by evaluating inflation-adjusted earnings over the past decade. Although introduced in the late 1980s, this valuation metric has been back-tested over 155 years, revealing that the historical average for the Shiller P/E is approximately 17.35. As of early 2026, the market is facing its second-highest valuation ever, exceeding a Shiller P/E of 40.
Historically, when the Shiller P/E has surpassed the 30 mark during a bull market, significant declines—ranging from 20% to 89%—in major stock indices have followed. Notably, it has only cross the 40 threshold three times in the last 155 years, including the current market period. The previous occasions involved severe declines of 49% during the dot-com bubble and 25% from January 2022.
Despite some recent volatility attributed to factors like geopolitical tensions—such as inflationary concerns related to the war in Iran—the overall market has remained resilient but costly. As of early April, the Shiller P/E ratio remained above 38, drawing further concerns among investors and analysts. Historically, no significant market downturn has concluded with a Shiller P/E exceeding 27, suggesting a potential for a substantial correction ahead.
As Jerome Powell nears the end of his tenure as Fed chair, his warnings regarding high equity valuations resonate strongly within financial markets, serving as a reminder for investors to heed the lessons of history.
