The S&P 500's historical earnings growth is ~6.7%. The ~9% growth of the last decade was an exception, driven by the unprecedented hyper-growth of a few mega-cap tech firms. As the law of large numbers catches up to these giants, investors should anticipate future index returns to revert to historical, lower norms.

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.

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.

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.

Current S&P 500 valuations, with near-record profit margins and a 26x multiple, make historical 10.5% annual returns mathematically improbable. Achieving this would require absurd P/E expansion to 43x or margin expansion to over 20%, suggesting a best-case scenario of only 5% annual returns.

A "Goldilocks" scenario of steady growth and disinflation could propel the S&P 500 to 8,000 by early 2026. This isn't a bubble prediction; rather, the market's structural shift to higher-margin tech companies means such a level would represent fair value, not dangerous overvaluation.