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.

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Active managers are struggling against the S&P 500 not just from bad picks, but because the market is dominated by a few AI stocks they can't fully concentrate in. Many also became too defensive during April's volatility, causing them to miss the subsequent sharp market rebound.

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.

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 market is more fragile than during the dot-com bubble because value is even more concentrated in a few tech giants. Ten companies now represent 40% of the S&P 500. This hyper-concentration means the failure of a single company or trend (like AI) doesn't just impact a sector; it threatens the entire global economy, removing all robustness from the system.

History shows that markets with a CAPE ratio above 30 combined with high-yield credit spreads below 3% precede periods of poor returns. This rare and dangerous combination was previously seen in 2000, 2007, and 2019, suggesting extreme caution is warranted for U.S. equities.

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 underperformance of active managers in the last decade wasn't just due to the rise of indexing. The historic run of a few mega-cap tech stocks created a market-cap-weighted index that was statistically almost impossible to beat without owning those specific names, leading to lower active share and alpha dispersion.

The current market is not a simple large-cap story. Since 2015, the S&P 100 has massively outperformed the S&P 500. Within that, the Magnificent 7 have doubled the performance of the other 93 stocks, indicating extreme market concentration rather than a broad-based rally in large companies.

Despite record market highs, the S&P 500's underlying earnings per share (EPS) have not yet recovered to their peak from early 2022. This "narrative violation" points to a hidden earnings recession for large-cap stocks, a fact that has been masked by market enthusiasm and multiple expansion.

The global economy's reliance on a few dominant tech companies creates systemic risk. Unlike a robust, diversified economy, a downturn in a single key player like NVIDIA could trigger a disproportionately severe global recession, described as 'stage four walking pneumonia.' This concentration makes the entire system fragile.