S&P 500 Concentration Has Hit 54%. History Suggests a Broadening Is Coming.

State Street’s holdings snapshot shows the S&P 500’s top 25 stocks account for 50.62% of the index weight as of July 2, 2026. Analyst Brian Belski rounds that figure to 54% and notes that this concentration level hasn’t been seen in roughly two decades.

Belski cites 1997 as a baseline: the top 25 stocks represented about 35% of the index then, held relatively stable near that level for 20 years, then surged to today’s 54%. That compressed gain matters because extreme concentration has a historical counterpart: broadening. When a handful of stocks dominate index returns, markets eventually rotate into smaller and midcap names. The Russell 2000 (IWM) is already signaling the shift. Through July 6, 2026, IWM returned 21.05% year-to-date, beating the S&P’s 10.46%. The pattern is testable. The questions are how long broadening lasts and what would accelerate it.

Key facts at a glance

• State Street data showed the top 25 S&P 500 stocks at 50.62% of index weight as of July 2, 2026, per State Street SPDR holdings.
• The top 5 stocks alone represented 25.75% of the index; the top 10 accounted for 36.40%.
• Russell 2000 (IWM) returned 21.05% year-to-date through July 6, 2026, outpacing the S&P 500’s 10.46% return, per Yahoo Finance.
• The S&P 500 Equal Weight Index, described by S&P Dow Jones as an equal-weight version of the S&P 500, returned 12.18% year-to-date, placing it between cap-weighted SPY and Russell 2000 outperformance.

How it works

The S&P 500 is a market-cap-weighted index. The 500 largest U.S. public companies are included, but the biggest companies drive the index mathematically. If Microsoft, Apple, and Nvidia all rise 2% in a day, the S&P 500 rises more than if Amazon, JPMorgan, and Tesla rise 2%, because of market-cap differences.

When a small number of companies grow much larger than the rest, market-cap weighting naturally creates concentration. All index funds passively track the weights, so money flows into the dominant names automatically. The 10 largest stocks in today’s index, as of July 2, represent 36.40% of the S&P 500.

Broadening is the reverse. When the largest stocks underperform or trade sideways, smaller names catch up. Index funds rebalance passively: they sell the winners (which decline in weight) and buy the laggards (which increase in weight). Active money often rotates into cheaper, overlooked parts of the market at the same time. The result is a cycle where concentration and broadening alternate.

A worked example: concentration is not a size story

Here’s Belski’s key insight. The Russell 2000 (IWM) is outpacing the S&P 500 this year. A naive explanation: small-cap stocks are in favor, so all small-cap indices should outperform. That would predict that smaller stocks within the S&P 500 itself, the SMID portion, should also outpace the index. But that’s not happening.

Belski observes the opposite. The top decile and mega-cap stocks continue to drive S&P 500 returns, while the Russell 2000 crushes the S&P 500 Equal Weight Index (RSP). This paradox reveals the mechanism. The Russell 2000 outperforms because it has completely different sector exposure than the S&P 500. It includes many unprofitable companies, high-beta names, and non-mega-cap cyclicals.

When money rotates away from mega-cap tech dominance, those Russell holdings accelerate. But the S&P 500’s internals remain driven by its largest names. Size isn’t the divide. Sector is.

The comparison below uses adjusted closes through July 6, 2026 from Yahoo Finance. It tests whether broadening shows up in the cap-weighted index, the equal-weight index, or the small-cap basket.

Index	YTD return, 2026	Weight style	Evidence tested
S&P 500 (SPY)	10.46%	Market-cap weighted	Mega-cap leadership; concentration visible
S&P 500 Equal Weight (RSP)	12.18%	Equal-cap weighted	Broad participation if all stocks counted the same
Russell 2000 (IWM)	21.05%	Market-cap weighted small-caps	Small/midcap demand; different sector mix

The equal-weight index (RSP) beat cap-weighted (SPY), suggesting some broadening is underway. But the Russell 2000’s outperformance is sharper, revealing a sector shift. When the S&P 500’s mega-cap concentration finally cools, SMID stocks within the S&P itself should accelerate further.

What would favor broadening

Broadening becomes more likely in specific market conditions, though Belski doesn’t provide a precise trigger. Historically, shifts from concentration to broadening have been driven by multiple expansions in smaller names, earnings surprises, or valuations mean-reverting. The mechanism varies.

The 10-year Treasury yield sat at 4.48% on July 1, 2026, per Federal Reserve data. Belski’s view: if Treasury yields decline below 4%, that would provide an additional tailwind for SMID stocks. Rising yields would pressure the opposite. But the relationship is not mechanical; earnings disappointments, fund rebalancing, or sentiment shifts can trigger rotation without waiting for a yield move. Timing is speculative. History shows broadening happens; it doesn’t show when.

Common misconceptions

“Broadening means all small-cap stocks outperform.” No. Broadening often means the lowest-quality small-cap names underperform while higher-quality SMID names participate. Russell 2000 includes unprofitable, low-revenue companies. When small-caps outperform, it’s often the less-beaten-down end of that market moving up, not the entire cohort.

“If Russell outperforms SPY, broadening is happening.” Not necessarily. Russell 2000 and S&P 500 have different sector exposures. Russell can run while S&P mega-caps remain dominant. True broadening occurs when smaller stocks within the S&P 500 catch up. The equal-weight index is a better gauge.

“Historical patterns guarantee future returns.” They don’t. Past concentration cycles may not repeat if passive index flows have structurally changed market behavior. Concentration could persist longer than history suggests, or broadening could arrive suddenly.

What this does not tell you

The historical pattern is backward-looking. It shows that concentration peaks have preceded broadening in the past. It does not prove:

• When broadening will occur. Concentration lasted longer in some cycles than others. Time horizons vary. An article written six months ago could have made the same broadening case, and the case would remain valid today. Timing is not implied.
• How long it will last. Broadening episodes can last weeks or quarters. No reliable formula exists for duration.
• Which small-cap stocks will lead. Broadening benefits some SMID names far more than others. Sector, profitability, and debt matter. A Russell 2000 rally does not transfer equally to every small-cap holding.
• Whether valuations will compress or earnings will accelerate. Broadening can occur via multiple expansion in smaller names, multiple compression in mega-caps, or earnings surprises. The magnitude of each driver changes outcomes.

FAQ

What does SMID mean?

SMID means small and midcap stocks. Here, it refers to companies below the mega-cap leaders that dominate the S&P 500 weight. The term is useful because a broadening market can lift smaller S&P 500 constituents, equal-weight baskets, and Russell 2000 names at different speeds. Same word. Different baskets.

Is the S&P 500 too concentrated today?

Yes, by historical standards. The top 25 stocks at 50% of index weight exceeds most of the past 20 years. Whether “too concentrated” triggers immediate rotation is a separate question. Concentration has persisted before, and it can persist longer if mega-cap earnings justify the valuations. The risk is that a catalyst, earnings miss, rate shock, or fund rebalancing, moves the needle quickly.

Why does the Russell 2000 outperform if broadening hasn’t occurred in the S&P 500 yet?

Russell 2000 and the S&P 500 are not the same. Russell includes 2,000 smaller public companies with different sector weightings, profitability profiles, and debt levels. When Russell outperforms, it reflects demand for those specific names, not for small-caps broadly within the S&P. True SMID broadening occurs when S&P 500 smaller names catch up, which is gauged better by the equal-weight index.

Is Belski’s 54% figure different from the State Street 50.62% data?

Yes. State Street’s most recent holdings file, updated July 2, 2026, shows the top 25 stocks at 50.62% of the SPY index. Belski’s 54% figure appears to be a rounded reference or a slightly different time window. Both are in the same ballpark and support the same thesis: current concentration is historically high.

How confident is the broadening thesis?

Moderate to high on the historical pattern; low to moderate on timing. The evidence is clear: extreme concentration has preceded broadening in the past. The uncertainty is whether today’s structural changes, passive index funds, mega-cap tech dominance, and AI narratives, have altered the pattern. A thesis that “broadening will occur eventually” is sound. A thesis that “broadening will occur in Q3 2026” is speculation.