The Hidden Concentration Risk in Market-Cap Weighted Index Funds
“Buy one index fund and you’re diversified across hundreds of companies” — that’s only half true. The holdings count is real, but how evenly your money is actually spread across them is a completely different question. When I first parked a chunk of my own portfolio in a broad index fund, I told myself, “500 companies, I’m covered.” A few years later I opened the fact sheet and actually added up the top 10 weights myself — and the number surprised me. Today I want to walk through why cap-weighted index funds concentrate the way they do, how concentrated things are right now, and a simple way to check your own fund.
”Diversified” Doesn’t Mean What You Think
The whole point of diversification is straightforward: spread your money so that trouble at one company doesn’t sink the whole portfolio, by letting independent single-company risks offset each other. As covered in why diversification reduces risk — and its limits, that effect comes from how evenly your money is spread, not from the raw count of holdings.
Here’s the catch: a market-cap weighted index fund was never designed to spread money evenly in the first place. Weights are set purely in proportion to market capitalization. Nobody sat down and decided “this feels balanced enough” — the fund simply mirrors whatever price tag the market has already assigned. So whether the fund holds 500 names or 3,000, that headline number alone tells you nothing about the quality of your diversification.
Why Cap-Weighted Indexes Concentrate By Design — and the History
There’s a self-reinforcing loop baked into cap weighting. When a stock’s price rises, its market cap rises with it. When its market cap rises, its weight in the index rises automatically. And a bigger weight means that stock has even more influence on the index’s return going forward. Nobody engineered this on purpose — it’s just the arithmetic of the methodology at work.
That’s why the top 10 holdings’ share of the index has swung wildly over time. According to RBC Wealth Management, the top 10 weight sat around 19% in 1990, climbed to roughly 23% by the end of 2000 as the dot-com bubble inflated (peaking intra-year near 27%), fell back to about 19% by the end of 2015, and has since climbed back to roughly 41% by the end of 2025.
Pinning down today’s exact figure to a single number is genuinely hard — it depends on the measurement date and methodology. Estimates range from about 32.5% as of May 2024 (per a Forbes tally) to roughly 41% at year-end 2025 (RBC/FactSet), with a separate Goldman Sachs calculation putting it around 33%. The precise number moves around, but the direction is consistent: somewhere between a third and over 40%, currently sitting closer to the upper end.
What’s striking is the dot-com peak of roughly 27% — a figure that shows up consistently across multiple analyses, which lends it credibility. Return contribution tells an even sharper story. Per MSCI, the top 10 stocks in the MSCI USA Index accounted for about 53% of the index’s return between March 2023 and February 2025. During the dot-com run (March 1998 to February 2000), that figure hit roughly 67%. Today isn’t quite at that extreme, but it’s headed in the same direction.
Three Ways to Diagnose Your Own Fund’s Concentration
Market-wide figures are just context — what you actually want to know is how concentrated your fund is. Three approaches, from easiest to most rigorous:
1. Total weight of the top 10 holdings
The simplest check. Pull up your fund’s fact sheet (most providers publish one monthly) and add up the percentages listed under “Top 10 Holdings.” It takes about a minute — and until I did this myself for the first time, I was just guessing.
2. Weight of the single largest holding
Same fact sheet, but look at just the #1 position. You don’t need to memorize company names — what matters is the method: how much of your portfolio rides on one company. If that number keeps climbing year over year, concentration is building.
3. Effective number of holdings (Effective N = 1/HHI)
The most rigorous approach. The Herfindahl-Hirschman Index (HHI) — originally a tool the U.S. Department of Justice uses to measure market concentration in antitrust cases — is calculated by converting each holding’s weight to a decimal, squaring it, and summing across every holding. Its inverse (1/HHI) tells you the “effective” number of equally weighted holdings your portfolio actually behaves like.
Running the numbers makes it click:
| Scenario | Composition | HHI | Effective holdings (1/HHI) |
|---|---|---|---|
| A. Evenly spread | 5 holdings at 20% each | 0.20 | 5.0 |
| B. Concentrated | 1 holding at 60% + 4 at 10% each | 0.40 | 2.5 |
| Actual S&P 500 (year-end 2024) | 500 nominal holdings | — | ~46 |
Scenario A splits five holdings at exactly 20% each: HHI = 5 × 0.2² = 0.20, so the effective count is 1/0.20 = 5.0 — matching the nominal count exactly. Scenario B has one holding hogging 60%: HHI = 0.6² + 4×0.1² = 0.36 + 0.04 = 0.40, dropping the effective count to 1/0.40 = 2.5. Both “hold” 5 positions on paper, but their real diversification differs by a factor of two.
So what’s the actual effective count for a fund holding all 500 S&P 500 names? Per Bridgeway’s analysis (Berkin & Liu), it was roughly 46 as of year-end 2024 — the lowest reading in 55 years. On paper you own 500 companies; from a risk standpoint, it behaves more like owning 46.
There’s a real limitation worth being upfront about: calculating an exact HHI requires the full weight distribution across every single holding, which isn’t something most individual investors can easily pull together. In practice, a simpler rule of thumb works fine: if the top 10 weight keeps trending up, the effective number of holdings is shrinking right along with it.
Equal Weight vs. Cap Weight: The Trade-Off
The obvious next question: why not just switch to an equal-weight index and sidestep concentration entirely? An equal-weight index assigns every stock the same weight — essentially building Scenario A above by design. Concentration drops, no question.
But it isn’t free. Keeping weights equal requires periodic rebalancing, which raises turnover and trading costs, and it tilts exposure toward smaller companies relative to the giants, which tends to raise volatility. Performance has flipped depending on the period, too. One secondhand analysis I’ve seen cited put equal-weight ahead by roughly 1.5 percentage points a year between 2003 and 2022, but cap-weight has clawed that back since 2023 as a handful of mega-caps carried the market. Neither approach wins consistently — don’t treat either as a permanent verdict.
None of this means today’s concentration level is a flashing crash signal. Per Goldman Sachs, 12-month returns following past concentration peaks have historically been positive more often than negative. There’s no reliable, consistent link between “concentration is high right now” and “returns next year will be bad.” This is worth understanding, not fearing.
What to Actually Do About It — A Checklist
- Once a year, open your fund’s fact sheet and add up the top 10 holdings’ weight yourself.
- If you buy a US-listed S&P 500 or similar ETF, don’t let it be your entire equity exposure by default — consider whether a broader total-market fund or an equal-weight fund belongs alongside it. See S&P 500 vs. total US market for a fuller framework on that specific choice.
- Keep this issue separate from geography. What we’ve covered here is concentration within an index by company; concentration by region (US vs. rest of world) is a completely different axis and worth checking on its own.
- For a broader look at assessing portfolio risk, how to evaluate investment risk and asset allocation basics are useful next reads.
- Trying to dodge concentration by hand-picking individual stocks tends to backfire. Index funds vs. individual stocks walks through the data on why.
FAQ
If I invest in an index fund, am I automatically diversified?
Only partly. You do cut single-company bankruptcy risk substantially, but because the fund is market-cap weighted, its weight is automatically concentrated in a handful of large companies. A high holdings count doesn’t guarantee even diversification.
What exactly is concentration risk?
It’s the risk that a portfolio’s performance is driven disproportionately by a small number of stocks or sectors. In an index fund, you can measure it with the combined weight of the top 10 holdings or with the effective number of holdings (1/HHI); the more concentrated it is, the more a handful of stocks drive your overall return.
Why do cap-weighted indexes end up concentrated in a few stocks?
Because weights aren’t assigned by a person — they’re set automatically in proportion to market capitalization. There’s a self-reinforcing loop: when a stock’s price rises, its market cap rises, and its index weight rises with it, so winners tend to keep growing their share.
How does today’s concentration compare with past periods like the dot-com bubble?
Per RBC’s analysis, the top 10 weight was about 19% in 1990, roughly 27% at its dot-com-era intra-year peak in 2000, back down to about 19% by year-end 2015, and around 41% by year-end 2025. Depending on the measurement date, estimates range from about 32% to 41%, but multiple sources agree that today’s level is close to past highs.
How is the effective number of holdings (1/HHI) calculated, and what does it mean?
HHI is the sum of each holding’s weight (as a decimal) squared, and the effective number of holdings is its inverse (1/HHI). Five holdings at an even 20% each stay at an effective count of 5, but if one holding takes 60%, the effective count drops to 2.5 even though there are still nominally 5 positions. The S&P 500 holds 500 names on paper, but its effective count was about 46 at year-end 2024.
If concentration keeps rising, is an equal-weight index safer? Are there other alternatives?
Equal weighting does lower concentration, but it comes with higher rebalancing costs and greater exposure to smaller, more volatile companies, so it isn’t automatically “safer” in every sense. Performance has flipped between the two approaches depending on the period. A reasonable approach is to keep cap-weighting as your core, consider pairing it with a total-market or equal-weight fund, and check the top 10 weight once a year.
The Bottom Line
- Holdings count ≠ real diversification. Cap-weighted indexes are built to concentrate in a handful of top stocks by design.
- The top 10 weight is roughly between a third and 40%+ today. Estimates vary by measurement date, but it’s near past highs.
- Checking your own fund takes about three minutes. Top 10 weight, top single holding, and effective holdings (1/HHI) are enough.
- Rising concentration isn’t a flashing crash signal. Historically, 12-month returns after past peaks were positive more often than not.
- Equal weighting is an option, not a cure-all. It trades concentration risk for rebalancing costs and volatility — go in with your eyes open.
Once I actually added up the numbers myself, I stopped saying “500 companies means I’m safe” quite so casually. There’s no need to fear concentration itself. Just make it a habit to check, once a year, exactly what you own and how much of it rides on a handful of names.