Your S&P 500 Index Fund Isn't as Diversified as You Hope

Capital at risk. Index funds fall in value along with the markets they track, and a concentrated index can fall further and faster than the number of holdings implies. Nothing here is personalised advice, and nothing here is an argument for abandoning index investing.

"I own five hundred companies" is one of the most comforting sentences in personal finance. It is also, on its own, close to meaningless. What matters is not how many companies you own but how much of your money each one represents — and in a market-capitalisation-weighted index fund, those two things can drift a very long way apart.

This is not a scandal, a defect, or a reason to sell anything. It is simply how the machine works, and understanding it is the difference between owning an index fund and knowing what you own.

How cap-weighting concentrates without anyone deciding to

A cap-weighted index holds each company in proportion to its market value. A company worth ten times another gets ten times the weight. Nobody at the index provider chooses to overweight the winners; the arithmetic does it automatically.

Follow that forward. When a company's shares rise faster than the market, its weight in the index rises too, so the index buys more of it — not by trading, but simply by holding what it already has as it grows. When shares fall, the weight shrinks. The result is a portfolio that continuously and mechanically increases its exposure to whatever has already gone up.

Over most periods this is a feature. It is precisely why cap-weighted indices are cheap to run, why they need almost no trading, and why they have historically been hard for active managers to beat. But in a period when a small group of very large companies rises far faster than everything else, the same mechanism quietly converts a broad index into something much narrower — and it does it without any announcement, any prospectus change, or any decision you were asked to approve.

How to measure it in your own fund, today

We are deliberately not printing a percentage for the top ten weight of any index. It changes daily, published figures go stale within weeks, and a number quoted from a third-party summary is exactly the kind of figure a reader acts on and should not. Measure it yourself instead. It takes about five minutes and the sources are free.

That single exercise replaces every article anyone can write about concentration, including this one. The number in front of you is current; anything quoted in prose is a snapshot of a day that has already passed.

What concentration actually does to risk

Three things, and they are worth separating.

It raises the effect of single-company events. A regulatory decision, a failed product cycle, an accounting problem or a founder departure at one of the very largest constituents moves the whole index in a way the same event at a mid-sized constituent never could. The index has hundreds of names but only a handful of names that matter to its daily path.

It concentrates a theme, not just a company. The largest companies in a market at any given moment usually got there through the same underlying story — an industrial boom, a consumer credit cycle, a technology platform shift. So the top of a cap-weighted index tends to be correlated with itself. Ten companies exposed to one theme are not ten independent bets; they are closer to one bet held ten ways.

It changes what your diversification is actually doing. If a large share of your equity is in one country's largest companies, adding a second fund of that same country's largest companies adds nothing. This is where concentration and home bias compound each other, and where most accidental portfolios go wrong.

What concentration does not do is make index investing a bad idea. The alternative — picking which of those companies to underweight — is stock-picking, and the evidence on how that turns out for most people is not encouraging. We lay it out in ETFs vs individual stocks and index funds vs active funds.

The four honest responses

The first row is a real answer, not a placeholder. Accepting the market as it is, at the lowest cost available, has served long-term investors well and requires no forecast about which of these companies is overvalued. The other three rows all involve a prediction, however mild, that the concentration will unwind.

Whichever you pick, the fee difference matters more than the elegance. An equal-weight fund that costs several times a cap-weighted one has to overcome that gap before its structural argument earns you anything: see fund costs: TER, tracking difference and spread.

The counter-argument, stated properly

The case for doing nothing is stronger than the worried version of this article suggests, and it runs like this.

Concentration is a description of the market, not a forecast about it. The largest companies are largest because a very great many buyers and sellers agreed on their value. Deciding to underweight them is a claim that you know something that price does not contain. That is an active bet, and it should be judged by the standards applied to any other active bet — which is to say, sceptically.

There is also a timing trap. Concentration has risen and fallen before, and the people who reduced their exposure to the largest companies because concentration was "historically high" have sometimes been early by many years, which in practice is indistinguishable from being wrong. If you tilt away from the market, you must be prepared to hold that tilt through a long stretch of it not working, which brings you straight back to the behavioural problem that ruins most portfolios: see the psychology of investing.

What to check, and what to ask

The honest verdict

An index fund does what it says: it owns the market in the proportions the market exists in. If the market has become concentrated, your fund has become concentrated, and that is the fund working correctly rather than failing.

The mistake is not owning it. The mistake is believing that the number of holdings on the label is a measure of safety. Go and read your fund's top ten weight, write it down, and decide — once — whether you are content with it. Then get back to the part that actually determines your outcome, which is how much you put in and how consistently.

About this article

This article was produced by NorwegianSpark Editorial — written with AI assistance and reviewed by the NorwegianSpark SA editorial team. YieldNav is operated by NorwegianSpark SA (org. 834 984 172), founded by Thomas Løvås Lokøy and Øyvind. We are not licensed financial advisers, and nothing here is personalised advice. Read our about page and affiliate disclosure.