The S&P 500 holds 500 large U.S. companies, weighted by market capitalization. That sounds diversified, and historically it has been. But after years of concentrated gains in a small group of large technology companies, the top 10 holdings of the S&P 500 now make up roughly 36% of the entire index (as of May 2026), compared to a long-term historical norm closer to 20% (the average across 1990-2016).

Why this happens mathematically

When a stock rises faster than others, its market cap grows faster, which means it takes up a bigger slice of any market-cap-weighted index. Years of strong tech outperformance have caused the largest companies to drift to a much bigger share of the index. The same math that makes index funds so simple to run also makes them more concentrated when winners keep winning.

What it means for diversification

• If you own only a U.S. total-market or S&P 500 index fund, you have less diversification today than the same fund offered 10 years ago.
• A single bad earnings season for a few mega-cap tech names can move your entire portfolio more than it would have historically.
• Sectors outside technology (healthcare, energy, consumer staples, utilities) collectively make up a smaller share of the index than they used to.

Approaches investors discuss in concentrated markets

• Equal-weight S&P 500 funds hold the same 500 companies but give each a roughly 0.2% slice. The mega-caps don't dominate.
• International stocks (developed and emerging markets) reduce U.S.-specific concentration.
• Small-cap and mid-cap exposure adds size diversification beyond the mega-caps.
• Doing nothing is also an approach. Concentration tends to cycle, and the index rebalances naturally over years as winners and losers shift.

What concentration is not

Concentration is not a flaw in an index fund, and it is not something the fund manager chose. A market-cap-weighted index is a rule, and the rule says hold each company in proportion to what the market says it is worth. If a few companies become an unusually large share of the market's total value, the index reflects that, because reflecting it is the entire job. A fund that refused to would no longer be tracking the index.

That matters for how you read the word. Concentration is a description of the market, arrived at by arithmetic, not a decision anyone made on your behalf and not evidence that anything is broken.

How concentration unwinds, when it does

There is no mechanism that forces concentration down, which is why it can persist far longer than commentary expects. It falls in one of two ways. Either the largest companies stop outgrowing the rest, so the gap stops widening and slowly closes as the broad middle catches up, or the largest companies fall, which drags the index down while reducing its concentration at the same time. The second route lowers the number in a way nobody enjoys.

Both routes are gradual in most periods and neither announces itself. This is the part that makes concentration a poor trigger for action: the measure can stay elevated for years, and the reading that tells you it is unwinding looks identical, at the start, to ordinary noise.

The ownership-literacy lens

If you hold a broad U.S. index fund, you already own this concentration, and you owned it before reading about it. The useful consequence is not a change to what you hold; it is knowing what your balance is actually made of. A fund labeled as five hundred companies behaves, on any given day, quite a lot like a much smaller number of them, and that explains why your account can move sharply on news about a handful of firms you never chose.

The same lens cuts the other way. When the broad middle of the market outruns the giants, a cap-weighted fund captures that too, just in the smaller proportion its weighting assigns. Knowing which version you own is accuracy about your own position, not a signal to do anything with it.

What this is NOT

This is not a prediction about AI companies, technology stocks, or the index as a whole, in either direction. It is not advice to buy, sell, hold, or avoid any security, sector, or fund, and no company is named here as a recommendation or a warning. Concentration is a measurable feature of how an index is built, not a judgment that the concentration is wrong or that it will unwind. This is not financial advice.

Sources

• S&P Dow Jones Indices, S&P 500 methodology: https://www.spglobal.com/spdji/en/indices/equity/sp-500/
• State Street Global Advisors, SPDR S&P 500 ETF Trust (SPY) holdings: https://www.ssga.com/us/en/individual/etfs/spy-spdr-sp-500-etf-trust