Skip to navigationSkip to main contentSkip to right columnADVERTISEMENTJeff Siegel, The Motley FoolSun, June 28, 2026 at 7:55 PM GMT+2 4 min readThe S&P 500 has delivered strong gains over the past several years. Artificial intelligence remains a dominant investment theme, and stock valuations have climbed well above historical averages. Whenever markets reach these levels, discussions about an impending crash tend to follow -- and with good reason.Indeed, history offers a useful perspective.Missed Nvidia in 2009? This Rare Signal Is Flashing Again. In 2009, a "Double Down" signal flashed for a little-known chipmaker called Nvidia. For the first time in years, that same "Total Conviction" signal is flashing for a company 1/100th the size of Nvidia. Continue »Image source: Getty Images.The CAPE ratioOne of the most widely followed valuation measures is the Shiller P/E CAPE ratio, which compares stock prices to average inflation-adjusted earnings over the previous 10 years. Historically, elevated CAPE ratios have been associated with lower long-term returns and, in some cases, major market corrections. And today, the CAPE ratio remains elevated.That doesn't mean a crash is imminent, though.History shows that expensive markets can remain expensive for years. In fact, the CAPE ratio first moved above its long-term average in the mid-1990s, yet the market continued rising for several more years before the dot-com bubble eventually burst.Market concentrationAnother indicator attracting attention is market concentration. A relatively small group of technology companies now accounts for an unusually large share of the