The artificial intelligence boom hasn't just sent stock prices soaring—it's completely rewired the U.S. equity market. And that's setting off alarm bells for some.

According to a new report from the Securities Industry and Financial Markets Association (SIFMA), concentration in the S&P 500 Index has reached historic levels, with high-growth tech stocks commanding outsized weight despite a much smaller share of corporate earnings.

Put simply: the top 20 stocks in the index are getting more expensive.

As of October 31, 2025, just 20 companies accounted for a staggering 50.8% of the index's total market value. That's up from 45.9% a year ago—and nearly double the 28% share those top names held in 2015.

But here's the catch: those same 20 companies only generated 9.1% of the S&P 500's total earnings.

In other words, half of the stock market is now riding on the backs of just a few companies—and their earnings have not caught up to their valuations.

The driver behind this shift is a white-hot surge in investor enthusiasm around artificial intelligence—and specifically, a small group of mega-cap names often referred to as the Magnificent 7: Alphabet Inc. (NASDAQ:GOOGL), Amazon.com Inc. (NASDAQ:AMZN), Apple Inc. (NASDAQ:AAPL), Meta Platforms Inc. (NASDAQ: