The S&P 500's Top 10 Stocks Are Near 40% of the Index. The Last Time That Happened Was 1965

October 20, 2026

Forty percent is the number worth sitting with for a second. That's roughly the share of the S&P 500's entire market value held by just ten companies, according to S&P Dow Jones Indices' own research. The index's own publisher says concentration at this level hasn't been seen since the mid-1960s. Numbers like this get thrown around a lot without context, so it's worth breaking "40%" down piece by piece: what it actually measures, where it came from historically, and what -- if anything -- it tells you about what comes next.

Photo: Arild Vågen, Federal Hall National Memorial, Wall Street, Manhattan (generic illustration of the U.S. financial district, not tied to this article's specific figures). Original file page, CC BY-SA 4.0

What does "top-10 concentration" actually measure?

The S&P 500 is weighted by market capitalization, so a handful of giant companies can carry a disproportionate share of the index's total value regardless of how many of the 500 names exist alongside them. "Top-10 concentration" simply adds up the weight of the ten largest constituents by market cap. It isn't a prediction or a valuation metric on its own -- it's a structural fact about how much of your return (and risk) in a plain S&P 500 index fund comes from a small group of companies rather than the other 490.

Put in dollar terms: a $10,000 position in a plain S&P 500 index fund today effectively behaves less like 500 roughly equal $20 slices and more like a single $4,000 stake spread across ten companies, plus $6,000 spread across the other 490. That doesn't make the fund riskier or safer by itself -- it just means the fund's performance on any given day leans much harder on those ten names than the "500 stocks" label suggests.

How concentrated is the index right now?

S&P Dow Jones Indices' own April 2026 research report, "Research in the Shadows of Giants," puts the ten largest S&P 500 companies at "almost 40%" of the index as of June 30, 2025. Separately, S&P Dow Jones Indices data reported by Reuters and Pensions & Investments puts the year-end 2025 figure at roughly 40%, up from about 19% ten years earlier at year-end 2015 -- meaning the top-10 share has roughly doubled in a decade. The report describes the current run-up as accelerating from 2023 onward, after pausing briefly in 2022 when large technology shares fell harder than the broad market. Unlike earlier concentration eras built on a mix of unrelated industries -- energy, pharmaceuticals, telecoms, banking -- today's largest holdings are linked by a shared theme: artificial intelligence infrastructure and the companies building or benefiting from it.

When was it last this high?

According to S&P Dow Jones Indices' own research, the last time concentration reached a comparable level was June 1965. The report's prose describes that group as "almost half of the index at the starting point," but the June 1965 weights in its own Exhibit 3 average 3.82% per company and sum to roughly 38% -- slightly below today's near-40%, not above it. The two readings of the same exhibit are worth naming rather than smoothing over; either way it is the same order of magnitude, and it is the closest historical precedent the index's own publisher points to.

Metric	June 1965 cohort	Today (year-end 2025 / mid-2025)
Top-10 weight of S&P 500	About 38% (sum of Exhibit 3 weights; the report's prose says "almost half")	Almost 40%
Average weight per top-10 company	3.82%	Not disclosed at that granularity; reported combined concentration roughly doubled since 2015 (~19% to ~40%)
Industry mix of top 10	Spread across energy, pharmaceuticals, telecoms, banking and more	Concentrated around a shared AI infrastructure theme
What happened over the following decade	1.17% annualized price return, 1965-1975	Unknown -- this is the open question, not a forecast

Why does the top-10 share keep climbing on its own?

It's worth understanding the mechanism, because it isn't a conspiracy or a design flaw -- it's how cap weighting is supposed to work. S&P Dow Jones Indices' own research notes that cap weighting lets tomorrow's leaders grow their index weight automatically as their market value grows, with no turnover or rebalancing decision required. A company that compounds faster than the rest of the index for several years in a row simply becomes a larger slice of it, year after year, without anyone "adding" it to a bigger position. That's exactly how the top-10 share moved from roughly 19% at year-end 2015 to roughly 40% a decade later: not one event, but several years of a small group of companies compounding earnings and market value faster than the other 490 combined.

What happened to the index after the 1965 peak?

This is the part worth sitting with longest. S&P Dow Jones Indices' own data shows the S&P 500 returned just 1.17% annualized (price return) over the ten years from 1965 to 1975 -- a weak decade that followed directly from that concentration peak. Three of the companies in the 1965 top-10 cohort eventually went bankrupt. And yet, measured over the full sixty years from June 1965 to June 2025, the index returned 7.42% annualized -- close to its long-run historical norm. The report's own framing calls this "the opposite" of what you might expect from a concentration warning, "albeit after a rocky start." Concentration fell as 1960s leaders faltered, then rose again later as new industry leaders emerged and grew -- which is itself the normal, repeating mechanism of a cap-weighted index, not a one-time anomaly.

To make the historical range concrete rather than abstract: if a $10,000 S&P 500 index investment made today matched the weak 1965-1975 stretch -- 1.17% annualized for ten years -- it would grow to about $11,234. If it instead matched the fuller 1965-2025 sixty-year average of 7.42% annualized, the same $10,000 over ten years would grow to about $20,457. That's not a prediction of either outcome; it's a way of sizing how wide the historical range actually is when concentration has been this high before. Nobody -- including S&P Dow Jones Indices' own researchers -- claims to know in advance which decade a new concentration peak resembles.

Does any of this change what a plain index fund holder owns?

This is general market history and education, not a recommendation to buy, sell, or avoid any security. If you hold a plain S&P 500 index fund, the practical takeaway is simply to understand what you already own: a meaningfully larger share of your return and risk is tied to a small number of companies than it was a decade ago, and that concentration has historical precedent that cuts both ways -- a rough decade followed by a strong six decades overall. Whether to adjust your own allocation in response is an individual decision that depends on your time horizon, risk tolerance, and overall portfolio, and is worth discussing with a qualified financial professional rather than deciding from a single statistic. Investors who want less top-10 exposure without abandoning U.S. stocks entirely sometimes look at equal-weighted index funds, which rebalance toward giving all 500 companies roughly the same weight -- though that approach carries its own trade-offs in cost, turnover, and historical performance that are beyond the scope of this piece.

Is there a case that this time is different?

The honest answer is that nobody knows yet, and treating either direction as settled would be overconfident. The case for "different" rests on today's largest companies posting real, substantial earnings and cash flow -- unlike several 1960s-era conglomerates whose growth stories outran their fundamentals before they collapsed. The case for caution rests on the fact that today's concentration is clustered around a single technology theme rather than spread across unrelated industries, which could mean a shared vulnerability if that theme's growth assumptions don't hold. S&P Dow Jones Indices' own research found only a "mildly positive" correlation between concentration levels and subsequent index performance -- the two sometimes move in opposite directions entirely, which is itself evidence against using concentration as a standalone timing signal in either direction. Concentration also isn't a fixed state: it can fall just as mechanically as it rises, the moment a leading company's growth rate slips below the rest of the index's for a sustained stretch. That happened briefly in 2022, when the largest technology names fell harder than the broad market and concentration paused before resuming its climb in 2023. Nothing about today's near-40% level locks in a particular direction from here.

Sources

This article is general investment education, not personalized financial advice. It does not recommend buying, selling, or holding any specific security. Historical and hypothetical figures are illustrative, not predictive, and past performance does not guarantee future results. Data as of the dates cited above; consult a qualified financial professional before making investment decisions. As of October 20, 2026.

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