Diversification is the closest thing investing has to a free lunch: spread your money across many holdings, and no single company blowing up can sink you. But "many holdings" and "well spread" are not the same thing, and a simple screen called the 5% test helps tell them apart, as NerdWallet explains.
What the 5% test is
The idea comes from US fund law. Under the Investment Company Act of 1940, a fund that wants to label itself "diversified" has to satisfy what is often summarized as the 75-5-10 rule: at least 75% of its assets spread across other companies, no single holding above 5% of assets, and no more than 10% of any one company's voting shares. The "5%" in that formula is the piece worth remembering: in the classic definition of a diversified fund, no one stock should dominate.
There are important caveats. The test is applied at the time a fund makes an investment, not continuously, so winners are allowed to grow beyond 5% afterward. And a fund can simply register as "non-diversified" and skip the limit altogether. So the 5% test is best used not as a legal gotcha but as a plain-English yardstick: if a single holding is a large share of a fund, that fund is more concentrated than the traditional standard implies.
Why it matters now
Apply that yardstick to the most popular index in the world and something jumps out. The S&P 500 is "market-cap weighted", meaning each company's slice of the fund is set by its total stock-market value, so the biggest companies get the biggest weights. After years of enormous gains in a few mega-cap technology names, that has left the index heavily tilted toward them. The group often called the "Magnificent Seven", Apple, Microsoft, Alphabet, Amazon, Nvidia, Meta and Tesla, has grown to more than 30% of the S&P 500, Forbes has reported, with individual names among them running well above the old 5% line.
That is not a scandal; it is just how cap-weighting works. But it means an investor who buys a standard S&P 500 fund for "diversification" is taking on more single-stock and single-sector risk than the headline number of 500 companies suggests. If a few of those giants stumble together, a supposedly broad fund feels it.
How to check your own funds
You can run the test yourself in a few minutes:
Look at the top 10 holdings. Every fund publishes a "fact sheet" listing its largest positions and their weights. If one stock is 7% or 8% of the fund, or the top 10 add up to 35%-plus, you are more concentrated than you might have assumed.
Check the sector weights. A fund can hold hundreds of stocks yet still have a third or more of its money in a single industry. A large technology tilt is the common one today. Concentration by sector is still concentration.
Look for overlap. If your broad index fund, your growth fund and your technology fund all count the same few mega-caps among their top holdings, you own those companies several times over without realizing it. Free portfolio "overlap" tools can surface this.
What to do with the answer
The point is not that concentrated funds are bad, or that you should chase a particular product. It is that "diversified" is a claim worth verifying rather than assuming. Once you have looked, you can decide whether you are comfortable with the mix. Some investors accept the tilt as the price of owning the market's winners; others balance it, for example by adding an "equal-weight" version of an index, which gives every holding the same slice regardless of size (at the cost of more frequent rebalancing), or by holding broader global and smaller-company funds.
Either choice can be reasonable. What the 5% test gives you is the information to make it on purpose, instead of owning far more of a handful of companies than you ever intended. This article is educational and general in nature, not individual investment advice.
