Few investors command more credibility than Warren Buffett. During the six decades he led Berkshire Hathaway (NYSE:BRK-A | BRK-A Price Prediction)(NYSE:BRK-B), shareholders enjoyed a cumulative return of roughly 6,099,294%! That works out to a 19.7% compound annual growth rate, compared with approximately 46,000% for the S&P 500, or 10.5% annually.
Yet Buffett has long argued that most investors should avoid stock-picking altogether. At Berkshire Hathaway’s annual meetings and in shareholder letters, he has repeatedly recommended a low-cost S&P 500 index fund as the best choice for most retirement savers.
And investors have listened. Funds are flowing into index funds at record levels. Vanguard’s S&P 500 ETF (NYSEARCA:VOO) recently became the first exchange-traded fund (ETF) in history to surpass $1 trillion in assets under management. But by following Buffett’s advice, investors are putting their retirement portfolios at significant risk.
Buffett’s Advice Helped Create a Trillion-Dollar ETF
According to Reuters, the S&P 500 ETF has attracted $69 billion of net inflows so far in 2026, following $118 billion in 2024 and $138 billion in 2025. No ETF has attracted more investor money this year.
Certainly, “buying the market” has looked particularly attractive, too. The S&P 500 completed nine consecutive weeks of gains last week, climbing to fresh all-time highs, and rewarding investors who stayed the course. While the index is on track this week to see that string broken, as artificial intelligence stocks sold off following Broadcom‘s (NASDAQ:AVGO) disappointing earnings report, the market is still up 10.7% in 2026.
Yet that very response shows why investors are putting themselves in danger. They believe they are getting broad market diversification, but in reality they are buying the same stocks that have sent the S&P to new heights. The numbers tell the story.
The Diversification Investors Think They Own
Investors buying VOO, SPDR S&P 500 ETF (NYSEARCA:SPY), or iShares Core S&P 500 ETF (NYSEARCA:IVV) often believe they’re reducing risk by spreading their money across 500 companies.
And while they are buying those stocks, the reality is more concentrated. According to MacroMicro data, the 10 largest stocks in the S&P 500 represented 37.5% of the index’s total market capitalization at the end of May. While that is down from the all-time high of 43% reached in March, it is still one of the greatest concentrations historically.
Consider, 10 years ago, the S&P 500’s top 10 stocks represented just 15.3% of the index’s market cap, and when Buffett told shareholders at Berkshire’s annual meeting five years later they would be better off buying index funds than individual stocks, they only represented 27.2%. The concentration has increased by nearly 38% since then.
Of those 10 stocks, seven of them are directly tied to the AI boom:
- Nvidia (NASDAQ:NVDA)
- Alphabet (NASDAQ:GOOG) and (NASDAQ:GOOGL)
- Microsoft (NASDAQ:MSFT)
- Amazon (NASDAQ:AMZN)
- Taiwan Semiconductor Manufacturing (NYSE:TSM)
- Broadcom
In other words, investors buying index funds to avoid concentration risk are increasingly buying the very same stocks driving AI enthusiasm and pushing market valuations higher.
What Happens If the AI Trade Slows?
Retirement investing is about protecting purchasing power over decades, not maximizing returns over a single year. That’s where concentration becomes a concern.
If AI spending continues expanding, index investors will likely benefit. But if corporate AI budgets slow, data center spending moderates, or earnings growth misses expectations, as occurred with Broadcom, the same stocks that powered the market higher could weigh heavily on index performance.
Regardless of how you look at it, an index where 10 stocks account for more than one-third of its value carries a different risk profile than one where those same stocks represented 15% a decade ago.
Buying the market used to mean buying broad diversification. Today, it increasingly means making a large bet on a handful of technology leaders.
Key Takeaway
In short, Buffett’s advice remains sound in principle, but investors should recognize how much the market has changed since he first championed index funds.
VOO, SPY, and IVV remain useful investment vehicles. However, they no longer provide the same level of diversification they once did because a small group of AI-driven giants now dominates the index.
When all is said and done, retirement investors shouldn’t assume that buying an S&P 500 fund automatically eliminates concentration risk. The data shows the opposite. Understanding what you actually own — not what you think you own — may be the most important retirement planning lesson of all.
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