The Shiller P/E ratio, a measure of long-term market valuation, has only crossed 40 three times: 1929, 1999, and today. The first two instances preceded major market crashes (The Great Depression, Dot-com Bust) and were followed by a decade or more of flat or negative real returns for investors.

Contrary to popular belief, earnings growth has a very low correlation with decadal stock returns. The primary driver is the change in the valuation multiple (e.g., P/E ratio expansion or contraction). The correlation between 10-year real returns and 10-year valuation changes is a staggering 0.9, while it is tiny for earnings growth.

During the bubble, a lack of profits was paradoxically an advantage for tech stocks. It removed traditional valuation metrics like P/E ratios that would have anchored prices to reality. This "valuation vacuum" allowed investors' imaginations and narratives to drive stock prices to speculative heights.

The CAPE ratio has crossed 40 for only the third time in 150 years. The previous two instances were immediately before the 1929 Great Depression and the 1999 dot-com bust, suggesting extremely negative 10-year returns for stocks.

A market enters a bubble when its price, in real terms, exceeds its long-term trend by two standard deviations. Historically, this signals a period of further gains, but these "in-bubble" profits are almost always given back in the subsequent crash, making it a predictable trap.

The CAPE ratio, which compares stock prices to average 10-year earnings, is at a level seen only twice before in history: just before the 1929 Great Depression and the 1999 dot-com bubble. This indicates a severely overvalued market ripe for a major correction.

During market bubbles and crashes, fundamentals become irrelevant. The price is dictated by the marginal 5% of highly-levered, less-sophisticated investors who buy at the peak and are the first to panic-sell, while the other 90-95% of shareholders hold steady.

Market bubbles evolve through predictable psychological stages. Phase one is buying an asset for its fundamental value. Phase two is using debt and leverage to acquire more of the appreciating asset. Phase three is pure speculation where investors, driven by greed, no longer care about the asset itself, only its potential for quick profit.

Instead of a vague label, Cliff Asness uses a rigorous test for a bubble: can you make the math work? He takes a stock like Cisco in 2000, assumes unprecedented growth for a decade, and if the valuation *still* doesn't make sense, he considers it a bubble.