June 28, 2026, 2:52 PM EDT. The S&P 500 has posted strong gains recently amid elevated valuations, particularly highlighted by a high Shiller P/E CAPE ratio, which measures stock prices against inflation-adjusted earnings over the past decade. Historically, elevated CAPE ratios often precede lower long-term returns and sometimes market crashes, though expensive markets can stay buoyant for years. A small group of tech giants like Nvidia, Microsoft, and Apple now dominate the S&P 500, raising concerns about market concentration risks. Past periods of concentrated market leadership, such as the 1970s "Nifty Fifty" and the late 1990s dot-com bubble, ended in significant declines. However, today's big tech firms boast strong profits and balance sheets, contrasting with past bubbles. Despite a stable economy with low unemployment and healthy corporate profits, the risk of market corrections remains, consistent with historical market behavior where 10% declines occur about once a year.