The top five companies in the S&P 500 account for a greater chunk of the index than at any time over the last 60 years.

Apple Inc. AAPL, Nvidia Corp. NVDA, Microsoft Corp. MSFT, Amazon.com Inc. AMZN and Google owner Alphabet Inc. GOOGL GOOG accounted for 29% of the S&P 500 at the end of 2024.

Jim Bianco, the president and index manager at Bianco Research Advisers, shared a chart on the social-media service X with the chart of the top five stocks, and the additional sarcastic comment: “I’m sure it’s nothing.”

The issue of stock-market concentration has many analysts worried.

Strategists at J.P. Morgan, led by Thomas Salopek, said the issue of concentration is something to watch. They said the recent drop in stock-market breadth since October is reminiscent of 2015, when stocks moved sideways initially as the percent of stocks above their 100-day moving average went from about 80% to 35% — and the market then corrected in August.

Looking at the difference between the S&P 500 and its equal-weight version, the J.P. Morgan strategists said it was the worst dislocation in decades.

But not everyone is worried.

Jeffrey Kleintop, chief global investment strategist at Charles Schwab, responded to Bianco’s message. “‘Winner take all” paradigms eventually end. But the end is hard to time,” he said.

“Rich valuations and high concentration in U.S. equity benchmarks are frequently cited concerns but may not represent the near-term risks many investors fear. Rather than reducing exposure to U.S. equities, we prefer managing around these risks, leveraging a combination of targeted exposures, active management and explicit hedges,” added strategists at BlackRock led by Gargi Chaudhuri.

Also read: The market’s wobbling. Here are BlackRock’s shields for the biggest risks.

Also read: The stock market is at the same valuation as when Greenspan made his ‘irrational exuberance’ comment