The S&P 500's quiet concentration problem
Index funds are diversified by design, but the major U.S. indexes are now more concentrated in a handful of names than they have been in decades. Here is what that means for your portfolio.
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 35% of the entire index — well above the historical average of 20-22%.
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.