Historically, US earnings outgrew the world by 1%. Post-GFC, this widened to 3%. Investors have extrapolated this recent, higher rate as the new normal, pushing the US CAPE ratio to nearly double that of non-US markets. This represents a historically extreme valuation based on a potentially temporary growth advantage.

With the S&P 500's Price-to-Earnings ratio near 28 (almost double the historic average) and the Shiller P/E near 40, the stock market is priced for perfection. These high valuation levels have historically only been seen right before major market corrections, suggesting a very thin safety net for investors.

Today's high S&P 500 valuation isn't a bubble. The market's composition has shifted from cyclical sectors (where high margins compress multiples) to mature tech (where high margins expand them). This structural change supports today's higher price-to-sales ratios, making the market fairly valued.

The historical average P/E ratio for U.S. stocks was supported by real GDP per capita growth exceeding 2%. With this growth rate having halved to around 1% over the last 25 years, the fundamental justification for a long-term P/E of 16-17 is weakened. A lower aggregate growth rate logically warrants a lower average valuation multiple.

Current market multiples appear rich compared to history, but this view may be shortsighted. The long-term earnings potential unleashed by AI, combined with a higher-quality market composition, could make today's valuations seem artificially high ahead of a major earnings inflection.

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.

While the S&P 500's price-to-earnings ratio is near dot-com bubble highs, the quality of its constituent companies has significantly improved. Current companies are more profitable and generate nearly three times more free cash flow than in 2000, providing some justification for today's rich valuations.

J.P. Morgan data shows that buying the S&P 500 when its P/E ratio is 23 has consistently led to 10-year annualized returns between -2% and 2%. This suggests investors should seek alternatives when the market is overheated.

While the S&P 500's PEG ratio (P/E to Growth) appears reasonable, this valuation is propped up by highly optimistic 23% forward earnings growth projections. This growth is four times expected nominal GDP, a historically unusual pace, making earnings delivery the key market risk.