Viewpoint · Myth check · Investing
Your Index Fund Has a Concentration Problem Too
Index fund concentration surprises people who bought a broad fund to spread their money out. The biggest companies get the biggest slices, and their moves become your moves.
The position
A cap-weighted index fund puts its biggest slices in its biggest companies; know that weight before you add the same names on top.
- Top-ten weight
- 35%
- Top-ten fall
- 20%
- Fund loss
- 7%
The fund fact sheet has a small table near the top: the ten largest holdings, each with a percentage beside it. Add those percentages up. The total is how much of your broad, diversified, own-the-whole-market fund actually rides on ten companies, and for a fund that holds hundreds of stocks, the number is usually bigger than the word “broad” led you to expect.
The myth: a broad fund spreads your money evenly
Many investors picture an index fund as a basket with equal pieces. Five hundred stocks, a five-hundredth each. Most widely held index funds do not work that way.
A market-cap-weighted index, which is how the S&P 500 and most broad market indexes are built, gives each company a weight in proportion to its total market value. The largest companies get the largest slices. When they rise faster than the rest, their slices grow further with no rebalancing to pull them back, because the weighting simply follows the market’s own valuation of each company, so a long run in which a handful of giants outpace everything else leaves the fund more and more tied to those few names at exactly the point where they have become the most expensive part of it. That is the design. It is also where the concentration comes from.
The fund’s name does not change while this happens. Neither does the number of holdings. Only the weights move.
The sum on a hypothetical index
Take a hypothetical cap-weighted index of 500 stocks in which the ten largest holdings make up 35% of the fund. Suppose those ten fall 20% and everything else stays flat.
Seven percent off a “whole market” fund from ten names. The other 490 did nothing. In the equal-weight version, the same fall barely registers.
Neither fund is wrong. They are different bets. The cap-weighted fund is a bet that the market’s valuations are about right, with extra weight on its largest companies. The equal-weight index gives every name the same slice at each rebalance, which trims the winners and adds to the laggards on a schedule, and carries more exposure to smaller companies as a result.
Where to check your own fund
Weights change as prices move, and fact sheets are updated on a schedule, often monthly or quarterly. The figure you read is a snapshot. Many funds also publish full holdings on their own site more often, which is the better source if you want the current number.
Read the sector weights next. Ten large companies spread across ten sectors is one kind of risk. Ten large companies bunched in two sectors is another, and it means a sector-wide sell-off hits the fund’s biggest positions all at once.
Do the same for every fund you hold. Two funds with different names can hold the same large companies at similar weights, and a portfolio of three “different” broad funds can turn out to be one bet held three times.
The strongest objection: cap weighting is the point
This is the argument that matters. A cap-weighted fund owns the market as it is. If the largest companies are large because investors value them highly, then holding them in proportion is the neutral position, and anything else is a bet against the market’s own pricing. It is also cheap to run and asks for no opinions.
Agreed. The objection is correct about the fund. Where it runs out is the rest of the portfolio. The trouble starts when an investor who holds the fund also buys the same large names directly, because they are familiar, they have done well and they show up on every screen.
Run it on a hypothetical $120,000 portfolio: $100,000 in the fund above and $20,000 more split among stocks that are already in its top ten.
Nearly half the portfolio rides on ten companies. The investor thinks the index fund is the diversified core and the stocks are the satellite, and the arithmetic says the satellite is sitting on top of the core’s largest positions. The position concentration calculator adds up exposure across funds and single stocks for you.
Where the concentration stops being a problem
It stops for an investor who accepts market weights on purpose and adds nothing that overlaps. Owning the fund and only the fund, with the top-ten weight known and accepted, is a clear decision with a clear risk. The problem is the unexamined version.
It matters less when the fund is a small part of what you own. For the investor adding single stocks, the check is short: list the fund’s top holdings, list your direct ones, and see where they meet. How much any one company should be, counting both routes, is worked through in how much of your portfolio should one stock be.
A broad fund is broad in count. Check whether it is broad in weight.
People also ask
How do you check how concentrated an index fund is?
Open the fund's fact sheet or holdings page and find the top holdings table. Add up the weights of the ten largest positions. That total tells you how much of every dollar in the fund rides on those ten companies.
Is an equal-weight index fund less concentrated?
Yes. In an equal-weight version of a 500-stock index, each company starts at 0.2% of the fund, so the ten largest companies together are about 2% after each rebalance. The trade-off is more exposure to smaller companies and more trading to keep the weights equal.
Does buying single stocks on top of an index fund add risk?
It adds concentration when the stocks you buy are already among the fund's largest holdings. You then own those companies twice, once through the fund and once directly, and a fall in them hits both parts of the portfolio at the same time.