1862: A five hundred dollar note issued by the Confederate States of America. (Photo by MPI/Getty Images)
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Somewhere along the way, the S&P 500 stopped being a diversified portfolio of American business and became a very expensive bet on a handful of technology companies. The numbers are startling: the top 10 companies in the index now account for approximately 40-41% of the total market capitalization, a level that has not just matched but exceeded the concentration seen during the dot-com bubble of 2000, when the top 10 held around 27%. For every $100 invested in a standard S&P 500 index fund, roughly $40 is flowing into ten companies. The other $60 is spread across the remaining 490.
This has been the dirty secret of passive investing’s golden decade. Index funds have generated outstanding returns — the S&P 500 delivered 24% in 2024 and 16% in 2025 — but those returns have been increasingly driven by a narrowing pool of contributors. In the most recent 28-session rally between late March and early May 2026, Nomura found that just 10 stocks drove 69% of the index’s gains. Alphabet, Nvidia, Amazon, Broadcom, and Microsoft led the charge. The other 490 companies were, largely, along for the ride. Investors who believe they own a diversified representation of the U.S. economy when they hold an S&P 500 fund are holding a belief that is no longer consistent with how the index is actually constructed.
How This Compares to History
Context matters. Market concentration is not new. In the late 1990s, the Nifty Fifty dominated, and tech stocks briefly commanded extraordinary weights in the run-up to the dot-com peak. But as RBC Wealth Management’s “Great Narrowing” analysis from January 2026 noted, the current top 10 weighting of approximately 40% is roughly double the concentration seen in 2015-2016, when the top 10 accounted for around 20%. That doubling happened in a single decade, driven almost entirely by the AI-related appreciation of a few mega-cap technology names.
The historical pattern of extreme concentration is not encouraging for forward returns. Goldman Sachs’s David Kostin has written that “high concentration today portends much lower S&P 500 returns over the next decade than would have been the case in a less concentrated market.” Tema ETFs, citing Goldman Sachs’s own research methodology, estimated that current concentration levels imply a forward 10-year return on the S&P 500 of approximately -5% based on historical correlation between concentration and subsequent returns. Current concentration also implies index volatility above 20% — higher than most investors benchmarking to the S&P 500 would expect from a “diversified” index exposure.
Three Specific Risks
RBC Wealth Management identified three specific concentration-related risks that deserve particular attention. The first is idiosyncratic shock risk: with NVIDIA alone representing nearly 8% of the index, a single earnings miss, regulatory action, or export control tightening can meaningfully move the entire index. This is qualitatively different from 1990, when no single company commanded more than 2-3% of the index, and an individual stock event would have had limited index-level impact.
The second is the passive concentration trap: the mechanics of passive index investing create a feedback loop where passive inflows disproportionately support the largest stocks, increasing their weights and reinforcing performance leadership regardless of fundamentals. Every dollar flowing into an S&P 500 ETF is being allocated to Nvidia, Apple, Microsoft, and Amazon in proportion to their weight — not in proportion to their valuation attractiveness or earnings outlook. This dynamic works beautifully when those companies are outperforming and reinforces concentration. When it reverses, the process runs equally powerfully in the opposite direction.
The third risk is AI correlation: unlike past periods of high concentration, where the top 10 spanned genuinely unrelated industries, today’s leaders are closely linked by a common theme. A significant reset in AI sentiment — whether driven by capex-to-revenue concerns, regulatory action, or a competitor breakthrough — could hit several of the top 10 stocks simultaneously, in a way that would have been impossible when the index’s concentration was spread across oil companies, banks, consumer staples, and industrial manufacturers.
What to Do About It
The instinctive response — dump index funds and go active — is probably not the right one for most investors. Active management has its own well-documented shortcomings, and the majority of active managers underperform their benchmarks over long periods even in environments where concentration risk is elevated. The more productive response is to adjust the form of index exposure rather than abandon it entirely.
Equal-weight S&P 500 strategies — the Invesco S&P 500 Equal Weight ETF (RSP) being the most liquid — allocate approximately 0.2% to each constituent, eliminating the feedback loop that drives passive concentration. RSP has historically outperformed the cap-weighted SPY by roughly 1.5% per year from 2003 through 2022; it underperformed meaningfully during the Magnificent Seven-driven rallies of 2023-2025. Whether that underperformance reverses depends on whether the current concentration normalizes. Given the historical pattern, the probability of normalization over a 5-10 year horizon is high.
Complementary approaches include factor-based strategies that tilt toward value, quality, or dividend growth — all of which naturally reduce concentration in mega-cap growth — and sector-based diversification that maintains large-cap equity exposure while shifting weight away from the tech-heavy top of the cap structure. None of these solutions are free; they all involve tracking error and periods of underperformance relative to the cap-weighted benchmark. The question each investor must answer honestly is whether the concentration risk they are currently carrying is consistent with the diversification benefit they believe index investing provides.