Are Your Index Funds as Diversified as You Think?

For years, investors have turned to S&P 500 index funds as a simple way to spread money across hundreds of large American companies.

But the index has become increasingly concentrated in a small group of technology giants, raising questions about how diversified those funds really are.

As of mid-2026, the 10 largest companies in the S&P 500 accounted for about 40% of the index, according to S&P Dow Jones Indices. That is the largest share on record.

Seven companies — Apple, Microsoft, Nvidia, Amazon, Alphabet, Meta, and Tesla — make up roughly one-third of the index. The group is often called the “Magnificent Seven.”

NVIDIA alone represents about 12% of the index, according to the figures cited.

That means roughly 40 cents of every dollar invested in a typical S&P 500 fund is tied to just 10 companies. The remaining 490 companies divide the rest.

So far, that concentration has worked in investors’ favor.

According to InvestmentNews, the Magnificent Seven gained about 27% in 2025, compared with roughly 16% for the broader S&P 500. Their strong performance has helped drive much of the market’s growth in recent years.

The risk is that the same companies that lift the index can also pull it down.

A market-cap-weighted index gives the largest companies the greatest influence. As their values rise, their share of the index grows automatically. Investors who continue buying the fund may therefore become more exposed to those companies without deliberately choosing to do so.

That does not mean S&P 500 funds are poorly designed or unsuitable for investors. They still offer broad exposure to major U.S. companies across many industries.

But owning hundreds of stocks does not always mean each stock has an equal effect on performance.

A fund can contain 500 companies and still depend heavily on a small number of them.

That has led some financial advisers to recommend looking beyond large U.S. growth stocks when building a portfolio.

Nick Ruder, chief investment officer at Kathmere Capital, told CNBC that investors should “make sure the portfolios are sufficiently diversified outside the mega-cap growth segment.”

That could mean adding exposure to smaller companies, international markets, value stocks, bonds, or other investments that do not move in step with the largest technology firms.

Equal-weighted S&P 500 funds are another option. Those funds give each company roughly the same influence, instead of allowing the biggest companies to dominate.

Still, equal-weighted funds come with their own tradeoffs, including different fees, turnover, and performance patterns.

The larger lesson is that investors should look beneath a fund’s label.

An index labeled “broad market” may still be heavily concentrated in a few companies or a single sector. Reviewing a fund’s top holdings can show where the real risks lie.

For now, the dominance of the largest technology companies has rewarded many investors.

But concentration cuts both ways. A portfolio that feels widely spread may be leaning more heavily on a handful of stocks than its owner realizes.