Goldman Sachs forecasts low long-term S&P 500 returns (3-6.5% annually). The key reason is that today's high market concentration implies higher future volatility, yet investors aren't being compensated for this risk because current valuations are already historically high and likely to contract.

The predictable economic progress of the post-WWII era was an anomaly, not the norm. Yet, most modern financial tools, like Monte Carlo simulations, were built on assumptions from this unique period, making them potentially ill-suited for today's more uncertain and volatile world.

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.

The US equity market's recent 15-year outperformance is nearly double the historical average cycle of eight years. Forecasters like Vanguard predict international stocks are poised to outperform in the next decade, suggesting a market leadership reversal is statistically overdue and investors should diversify globally.

While politicians tout the S&P's rise, it's misleading. The US market ranks near the bottom (20th out of 21) of Western markets in recent performance. When factoring in the dollar's 10% decline against foreign currencies, the S&P has significantly underperformed its global peers in Europe and Asia.

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.

Contrary to standard finance theory, historical data across many countries shows no consistent equity risk premium. Stock and bond returns are driven by independent factors, meaning investors should analyze their potential returns separately rather than assuming stocks will automatically outperform bonds by a set margin.

The historical outperformance of stocks has a standard error so large (2.1% on a 5.4% premium) that the true premium could be anywhere from 1% to 9%. This statistical uncertainty makes history an unreliable guide for future returns.

Analyzing U.S. market history creates a false sense of security due to survivorship bias. After the U.S. and U.K., the next six largest markets in 1899—including Germany, France, and Japan—all went to zero at some point. This highlights the extreme risk in long-term, single-country bets.