The Vanguard S&P 500 ETF (VOO) is far and away the world’s largest exchange-traded fund, with $1.7 trillion in assets and a long-standing endorsement from Warren Buffett that has cemented it as a default holding for retail investors globally. But its market-cap-weighted structure has spawned mounting concentration risks most passive shareholders overlook — creating a stark divide between the industry staple and its equal-weight peer, the Invesco S&P 500 Equal Weight ETF (RSP).
Buffett’s backing for low-cost S&P 500 index funds is well-documented. In his annual letter to Berkshire Hathaway Inc. (BRK.A, BRK.B) shareholders, the billionaire investor laid out explicit instructions for his estate: allocate 90% of capital to a low-cost Vanguard S&P 500 fund and 10% to short-term government bonds. Berkshire itself built a position in VOO in 2019 and held it for years, before liquidating the entire stake in the fourth quarter of 2024, according to U.S. Securities and Exchange Commission 13F filings.
That pedigree has turned VOO into the archetypal “buy-and-forget” index investment. Yet the S&P 500’s core weighting methodology is quietly warping the fund’s risk profile, concentrating both upside and downside in a shrinking pool of megacap stocks.
Under the S&P 500’s market-cap framework, larger companies command an outsize share of the index. For VOO, that rule has translated to extreme overexposure to the technology sector, which has led market gains throughout the artificial intelligence boom.
Today, the fund’s top five holdings — Nvidia Corp. (NVDA), Apple Inc. (AAPL), Microsoft Corp. (MSFT), Amazon.com Inc. (AMZN) and Alphabet Inc. (GOOGL) — collectively make up nearly 28% of its total portfolio. The technology sector alone accounts for 38.6% of VOO’s assets.
That lopsided weighting has amplified returns during the tech rally, but it leaves investors deeply vulnerable to a single-sector downturn. Any meaningful pullback among the megacap tech leaders would ripple disproportionately across the broader index.
The concentration trend is poised to accelerate. Following SpaceX’s historic initial public offering, AI peers Anthropic and OpenAI are also on track to go public later this year. With the S&P 500 Index Committee retaining its 12-month waiting period for new listings, all three firms are set to join the benchmark in 2027.
At current valuations, SpaceX alone could represent 3% to 4% of the index. Combined with existing tech heavyweights, the sector’s share of the S&P 500 could surpass 50% by the end of 2027 — leaving passive VOO investors with nearly half their portfolio tied to tech, often without explicit awareness.
For investors looking to dial back concentrated tech exposure without leaving the S&P 500 universe, RSP offers a straightforward alternative. The fund applies an equal-weight methodology, assigning roughly identical weighting to each of the 500 index components, fundamentally restructuring the portfolio’s risk profile.
The gap in sector allocations is stark. Technology makes up just 19% of RSP — nearly 20 percentage points below VOO’s weighting. The reduced exposure to tech, communication services and consumer discretionary is redistributed evenly across eight other sectors: industrials climb from 8.3% in VOO to 15% in RSP; real estate jumps from 1.8% to 6.1%; and financials, healthcare, utilities and materials all see meaningful increases.
The result is a more diversified portfolio driven by a broader set of economic forces, rather than the ebb and flow of a single growth narrative. That balance has proven its value during market shifts.
History shows the two strategies excel in distinct environments. During extended bull markets, the cap-weighted VOO typically leads, riding the momentum of its largest holdings. But during corrections and style rotations, the equal-weight RSP demonstrates far stronger downside protection. In the 2022 bear market — the last year the S&P 500 posted an annual loss — VOO dropped 20%, while RSP fell just 13%. Through 2026, as markets rotate toward value and defensive equities, RSP has also notched a modest performance edge.
None of this undermines Buffett’s core advice: low-cost broad-market index funds remain the best long-term option for most individual investors. What it does underscore is that the S&P 500 is not a perfect proxy for the full U.S. stock market, and cap-weighted and equal-weight strategies serve very different purposes.
With AI valuations stretched and tech’s grip on major indexes only tightening, RSP’s equal-weight framework gives investors a simple way to spread sector risk, capture upside across a wider swath of the market, and buffer their portfolios against the next correction. For passive investors, it’s a timely reminder that even “set it and forget it” index investing requires knowing exactly what you own.
