You Own an Index Fund, So You're Diversified — Right?

Opinion · July 1, 2026 · 6 min read · By Elizabeta Dimoska

Buy a broad index fund and you're diversified. It's the first rule most of us learn, and it's mostly good advice — we've made that case ourselves more than once on this site. One fund, hundreds of companies, instant diversification, done.

Except the S&P 500 in 2026 quietly stopped being quite as diversified as the pitch implies. And if you own a plain S&P 500 fund — which for a lot of Canadian readers means VFV, and for US readers VOO or SPY — it's worth understanding what you actually hold. Because "500 companies" and "500 equal companies" are very different things.

(Quick note on what this is: an opinion piece, not a warning to sell anything. We think index funds are great. We also think you should know what's under the hood.)

The math most people never look at

The S&P 500 is market-cap weighted. That means it doesn't hold 500 companies in equal 0.2% slices. Instead, each company's share of the index is set by its size. The bigger the company, the bigger its slice — and the giants have gotten enormous.

Here's where things stand in mid-2026:

• The "Magnificent Seven" — Apple, Microsoft, Alphabet, Amazon, Nvidia, Meta, and Tesla — make up roughly 34% of the entire S&P 500. (The Motley Fool)
• The top 10 stocks now account for about 40% of the index — meaning the other 490 companies share the remaining 60%. (Lord Abbett)
• Nvidia alone is around 7.4% of the whole index — a single company worth more than most entire sectors. (ClaritX)

To put that in perspective: historically, the top 10 stocks averaged about 24% of the index, and the previous high-water mark was 28%, set back in 1970. (Forbes) We are now well above both. When you buy an S&P 500 fund today, roughly 40 cents of every dollar goes into just ten companies, most of them large tech names riding the same AI wave.

That's not necessarily bad. But it is very different from what "I own 500 companies" makes you picture.

Why concentration cuts both ways

Let's be fair to the other side of this, because it's a real argument. Concentration is why index investors have done so well lately. From 2023 through 2025, the market-cap-weighted S&P 500 gained about 68%, while an equal-weighted version of the exact same 500 companies gained roughly 34%. (Forbes) Letting your winners grow into giant positions roughly doubled your return over that stretch. Anyone who "diversified away" from the big names left a lot of money on the table.

And these aren't flimsy companies. Today's giants generate a huge share of the market's actual profits and cash flow — the top 10 produce something like 31% of all S&P 500 earnings. (Armstrong Fleming & Moore) This isn't the dot-com era of profitless hype. The concentration is built on real earnings.

So why bring it up at all?

Because the same math works in reverse

When a handful of stocks make up 40% of the index, the index's fate is tied to their fate. If they stumble together, they drag everything down with them — no matter how well the other 490 companies are doing.

We got a crisp demonstration of this in early June 2026, when the Magnificent Seven collectively shed about $2 trillion in market value in a matter of weeks. Because those seven names are more than a third of the index, their drop acted like a gravitational anchor, pulling the whole S&P 500 lower even as plenty of mid-cap and value stocks were quietly rising that same stretch. (ClaritX) Investors who thought they owned "the whole market" found out they mostly owned a bet on seven correlated tech stocks.

The subtler point: these seven move together more than they move with the rest of the market. They're exposed to the same themes — AI spending, data centers, interest rates. So the diversification you feel you have (500 names!) is partly an illusion. You have far fewer independent bets than the label suggests.

There's also a hidden double-up problem. If you own an S&P 500 fund and a Nasdaq-100 fund (like QQQ) and a US tech fund, you don't have three diversified holdings. You have the same dozen mega-caps three times over.

What I'd actually do about it

Not sell your index fund. This isn't that kind of article. But a few genuinely useful moves:

• Look up your real exposure. Log into your brokerage and add up how much of your total portfolio sits in the top 10 names once you account for overlap between funds. People are routinely surprised. (Our portfolio tool and stock comparison tool can help you see the overlap.)
• Know why you own what you own. If you're comfortable being ~40% weighted toward ten mega-cap tech-adjacent companies, own that on purpose. It's a defensible bet. Just don't hold it by accident while believing you're broadly diversified.
• Consider adding the pieces the S&P 500 underweights. International stocks, small- and mid-caps, and value stocks are the parts of the market a cap-weighted US index gives you the least of. A little goes a long way toward real diversification.
• Know that an equal-weight version exists. Funds like the equal-weight S&P 500 (RSP) hold the same 500 companies but in roughly equal slices, which dramatically reduces single-stock risk. It tends to lag when the giants lead and shine when the market broadens out. It's a tool, not a must — but it's good to know it's on the shelf. (Forbes)

The honest takeaway

Index funds are still one of the best inventions in personal finance, and "buy the index and hold" is still advice I'd give my own family. None of this changes that.

But the phrase "I'm diversified because I own an index fund" is doing a lot of quiet work these days. In 2026, the S&P 500 is the most concentrated it has been in over half a century. That has been a tailwind on the way up. The same concentration is what makes it fall harder when the leaders turn.

You don't have to do anything about it. You just shouldn't be surprised by it. The investors who get hurt in a downturn usually aren't the ones who took a known risk on purpose — they're the ones who didn't realize they were taking it at all.

RiskStock is educational, not financial advice. We're not licensed advisors, and nothing here is a recommendation to buy or sell any specific security. Always do your own research.

Sources: The Motley Fool — The Magnificent Seven's Market Cap vs. the S&P 500; Lord Abbett — Time for a Conversation About Stock Market Concentration; Forbes — S&P 500's Weight in Mag 7 Passes 30%: A Diversification Risk?; Armstrong Fleming & Moore — Market Concentration and the Magnificent Seven.