The Hidden Concentration Risk in The S&P 500
Most investors who own the S&P 500 Index believe they're broadly diversified. They may want to look more closely. A quiet but significant shift in the index's composition means that what many investors think they own and what they actually own may not be the same thing.
For decades, the S&P 500 gave investors broad exposure to corporate growth and a simple way to diversify risk. Rising concentration has changed that. Today, the top 10 stocks make up nearly 40% of the index’s market cap.1More of the market’s performance now depends on a narrow group of technology giants whose future growth is closely tied to AI.
We're not skeptical of these companies. Many of them are exceptional businesses that have earned their place. What gives us pause is the weight of expectation now sitting on top of them and how many other companies across energy, infrastructure, and beyond have quietly tied their own valuations to the same AI growth story. When that much of the market is moving on the same narrative, a shift in sentiment doesn't stay in one sector. It moves through the whole index.
Why index risk deserves a closer look
The S&P 500’s strong recent performance has pushed conversations about risk into the background. The index has delivered around 23% annualized returns from 2023 through 2025 and made a “buy the index” approach feel like a strategy.2 But when more of the index is driven by fewer companies tied to the same theme, what looks like a diversified portfolio may be carrying more concentrated risk than investors realize.
For individual investors, particularly those close to retirement or with specific financial goals, concentration risk can upend wealth plans. A significant drawdown at the wrong moment can reduce portfolio values, interrupt compounding, and force investors to sell assets at depressed prices. That’s easy to remember when markets are rough. But when the S&P 500 is riding high, attention to risk often fades.
Strong performance attracts more capital. So more capital flows into the index. And because the S&P 500 is market-cap weighted, which means larger companies receive larger weights in the index, more of that money goes to the companies that are already dominant. Those flows help the biggest winners grow even bigger.
Getting caught in the Slot Canyon
That concentration creates what we think of as the Slot Canyon.
The strong flow of capital attracts more investors. At first, joining the flow feels good. You see the index rising and wonder why you’d own anything else. But as the flow grows stronger, it carves a channel that keeps narrowing, and you find yourself carried into the same companies that are relying on the same market drivers.
As the speed picks up, the channel becomes a canyon and the walls around it get higher. By the time you realize how much of your wealth plan depends on the same narrow set of companies, there’s far less room to maneuver than you expected. It’s difficult to change course. You don’t know whether you’re heading for rapids you can navigate or a waterfall that could sink you.
High valuations and crowded trades signal rising risk
This isn’t the first time that investors have been caught in a Slot Canyon. During the Nifty Fifty era in the late 1960s and early 1970s, when investors crowded into a group of 50 prized growth stocks, many stockholders suffered. After the S&P 500 peaked in January 1973, it did not reach another price-index high until July 1980.3
It happened again at the peak of the dot-com bubble in 2000, when the top 10 stocks accounted for roughly a quarter of the S&P 500.4 Cisco, for example, briefly became the world’s most valuable company in March 2000, before its stock fell nearly 90% over the next two and a half years. It did not reclaim its dot-com-era high until December 2025 — 25 years later.5
In both cases, many of the underlying businesses were strong and profitable. Investors were not buying weak businesses. What eventually hurt them were high stock valuations and the speed at which a crowded trade can unwind when sentiment turns. Today's market concentration is higher than it was during the dot-com peak and, by some measures, approaches levels seen in the Nifty Fifty era — a period that preceded a significant market correction in 1973–74.6
We can’t predict the future, but trends can point us to where risk is rising. Hartford Funds research shows that when the top 10 stocks account for 30% or more of the S&P 500, the other 490 stocks have outperformed the top 10 in 84% of subsequent five-year periods.7 Early this year, J.P. Morgan Global Research warned that with more than 50% of the S&P 500’s market value now held in just 20 stocks, crowded market positioning could make even small institutional rebalancing flows produce outsized price swings.8 Concentration stretches valuations and weakens the market’s ability to absorb selling when sentiment turns.
For long-term investors, high market concentration raises the question of whether index exposure still underpins their wealth plan and provides the diversification and downside protection they expect.
Three key risk allocation questions
Risk allocation, not benchmark performance, should drive prudent wealth planning. Proper allocation helps a portfolio absorb downturns without losing its ability to compound over time. When reviewing risk allocation, three questions are essential:
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How much of the portfolio depends on a narrow group of stocks?
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How much of the portfolio moves with the same market drivers?
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How much damage could a reversal do to your long-term plan?
That review will show whether the index still supports your wealth plan, or whether concentration has quietly changed your exposure and pulled you toward the Slot Canyon.
Strong markets have a way of making discipline feel unnecessary. When concentration is winning and the index is climbing, diversification can look like a drag. But staying the course isn’t about ignoring the returns around you. It’s understanding that your portfolio exists to serve your life, not to mirror the market. The companies driving today's concentration are genuinely transformative. AI may well reshape the economy in ways that are difficult to overstate. The harder question is whether the market's assumptions about the pace and scale of that buildout are accurate. Valuations today are pricing at a sustained level of AI infrastructure demand. If that demand proves slower or more uneven than expected, the correction doesn't stay in tech. It moves through the whole index.
This pattern has played out before — in the late 1960s and late 1990s. Each time, what ultimately hurt investors wasn't that the companies were bad businesses. It was that the concentration made portfolios far more vulnerable to a single turning point. The investors who come through those moments in the best shape are those who understood what they owned and why before the current turned.
Frequently Asked Questions
Is the S&P 500 still diversified?
The S&P 500 still gives investors exposure to around 500 companies. But because it is market-cap weighted, the biggest companies make up the largest share of the index. That means its returns are heavily influenced by a small group of mega-cap technology stocks.
What is index concentration risk?
Index concentration risk is the risk that a small number of companies have an outsized effect on an index’s performance. In the S&P 500, the largest stocks now account for a much larger share of the index than most times in the past. That has increased concentration risk for investors who assume the index still provides broad diversification.
How much of the S&P 500 do its 10 largest stocks represent?
Today, the top 10 stocks make up nearly 40% of the S&P 500’s market cap. They have a major impact on how the index performs.
Why has the S&P 500 become more concentrated?
The S&P 500 is market-cap weighted, so larger companies receive larger weights in the index. When the biggest companies keep rising, more index money flows into them, which can make the index even more concentrated.
Why does index fund risk matter for long-term investors?
Index fund risk matters because many long-term investors use index funds inside retirement plans, wealth plans, and goal-based portfolios. As indexes grow more concentrated, these investors may inadvertently take on greater concentration and correlation risk. Correlation risk, specifically, is the risk that assets expected to behave independently will instead decline in tandem during market stress.
How does AI affect S&P 500 concentration risk?
AI has become a major growth story for several of the largest companies in the S&P 500. If many of those companies are tied to the same AI expectations, investors may face more correlation risk because their portfolio depends on the same market theme.
What should investors understand about their index exposure?
The key questions are how much of the portfolio depends on a narrow group of stocks, how much of it moves with the same market drivers, and how much damage a reversal could do to the long-term plan.
Is the S&P 500 diversified enough for a long-term wealth plan?
The answer depends on the investor’s goals, risk tolerance, and other holdings. The S&P 500 still gives broad market exposure, but rising concentration means investors should check whether that exposure is providing them with the diversification they expect.
How can a sustainable investing approach differ from index exposure?
An index fund tracks the market as it is weighted. A disciplined sustainable investing approach can look more closely at business quality, long-term risks, balance sheets, and whether each holding still supports the investor’s goals.
1“How extreme is market concentration?” J.P. Morgan Asset Management, May 20, 2026.
2S&P 500 Historical Annual Returns (1927-2026), Macrotrends.
3“How the S&P 500 Performed During Major Market Crashes,” Visual Capitalist, August 5, 2020.
4“Is today's AI boom bigger than the dotcom bubble?” Reuters, July 23, 2025.
5“Cisco Was the Dot-Com Bust’s Biggest Loser. Now It’s at a 25-Year High,” Barron’s, June 20, 2025.
6"Equity Outlook 2026: Tech-Tonic, A Broadening Bull Market," Goldman Sachs Research, January 2026; "The S&P 500 Is Expected to Rally 12% This Year," Goldman Sachs Insights, January 9, 2026; "The Great Narrowing: S&P 500 Concentration," RBC Wealth Management, January 23, 2026.
7“The Messy Aftermath of Concentrated Markets,” Hartford Funds, February 10, 2026.
8“How extreme is market concentration?” J.P. Morgan Asset Management, May 20, 2026.
DISCLOSURES: This material is provided for informational and educational purposes only and is not intended to constitute investment advice or a recommendation to buy or sell any security. The views expressed reflect the opinions of Reynders, McVeigh Capital Management, LLC (“RMCM”) as of the date of publication and are subject to change without notice.
References to specific industries, companies, historical events, or market conditions are provided solely to illustrate RMCM’s investment philosophy and research process and should not be interpreted as guarantees of future outcomes or performance. Past performance is not indicative of future results. Investing involves risk, including the possible loss of principal. Forward-looking statements are based on current expectations and assumptions and are inherently uncertain; actual results may differ materially. Sustainability-related considerations are one of many factors evaluated in RMCM’s investment process and may not be relevant or applicable for all clients or strategies.
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