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.

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.

Investors often mistakenly equate strong economic growth with strong stock market performance. Historical data, particularly China's market performance versus its GDP since 1992, shows no reliable correlation. Starting valuation is a far better predictor of long-term returns.

The current environment, where forward price-to-earnings multiples fall significantly while earnings growth remains strong (up over 20%), is a classic sign of a temporary correction within a larger bull market, not the start of a prolonged downturn.

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.

Contrary to a common myth, high equity valuations do not reliably revert to a historical mean. An analysis of 32 different valuation scenarios found only one case of statistically significant mean reversion. Structural economic shifts, like reduced GDP volatility since the 1990s, justify higher sustained valuation levels.

Across 200 years and 56 countries, the single most important factor for long-term investing success is the starting valuation. Buying portfolios with low P/E ratios or high dividend yields consistently outperforms buying expensive assets by 3-4% annually over the long run.