You probably own an S&P 500 index fund. Most people do. It’s the bedrock of retirement portfolios across America... and it was built on one promise: diversification. Five hundred stocks. Spread the risk.

Right now, that promise is stretched thinner than at any point in modern market history.

As of this summer, just 10 companies — Apple, Nvidia, Microsoft, Amazon, Alphabet, Meta, and a handful of others — make up 37% of the entire S&P 500. For every dollar you have in that fund, 37 cents rides on ten names. The other 490 stocks split the remaining 63 cents among themselves.

Now look at the chart. From 1995 through 2015, the top 10 held between 18% and 27% of the index. Even at the peak of the dot-com bubble — when every cabdriver had a tech stock tip — the top 10 reached about 27%. After that spike faded, concentration settled back into the high teens for over a decade. Today blows right past all of it.

And here’s the part that really gets me. Most people bought index funds specifically to avoid putting all their eggs in a few baskets. That was the whole point. But starting around 2017, the concentration began climbing... and it hasn’t come back down.

This doesn’t mean you sell your index fund tomorrow. These are real companies with real earnings — Alphabet and Tesla just reported, Apple and Amazon are up next week. But when 37 cents of every dollar in your portfolio depends on how a few tech CEOs perform this quarter... you ought to at least know that going in.

Pull up your portfolio this weekend. Check how much rides on just a handful of names. If the answer surprises you, that’s worth a phone call to your advisor. This week, while earnings are still coming in — not next month.