Nearly half of America’s most-watched stock index now rides on a single idea. AI-linked companies account for 49% of the S&P 500, the second-largest thematic concentration in the index’s history.
The only theme that has gone further was railroads, back in the 19th century.
Inside the 49%
The figure comes from JPMorgan, which tracks a basket of 41 companies tied to artificial intelligence. As of early October 2026, that group made up 49% of the S&P 500’s total market capitalization.
Jim Bianco of Bianco Research reached a similar conclusion with his own 41-stock AI basket. By mid-October, he found it held around 48.7% to 49% of the index.
The heavy hitters will be familiar to anyone with a brokerage app. Nvidia alone represents 8.21% of the S&P 500. Apple follows at 7.05%, then Alphabet at 6.13%, Microsoft at 5.65%, and Amazon at 4.02%.
A concentration story decades in the making
The AI basket has also done most of the heavy lifting this year. JPMorgan’s group drove roughly two-thirds of the S&P 500’s year-to-date increase in market capitalization.
The concentration shows up in other measures too. The S&P 500’s Information Technology sector hit a weight of approximately 39% earlier in 2026, surpassing the dot-com peak of 33%.
Meanwhile, the top 10 stocks in the index now account for 38-40% of it. That is the highest concentration seen since 1965.
The last time a single theme exceeded 50% of the S&P 500 was during the railroad era of the 19th century, when track-laying companies dominated US equity markets.
What this means for index investors
The research describes the S&P 500 as increasingly looking like two separate markets: AI-centric companies and everyone else.
The bull case rests on fundamentals, not just hype. Strong earnings growth and heavy capital investment in AI infrastructure have been supporting these tech stocks. The sustainability of these returns is increasingly uncertain for passive S&P 500 investors, who cannot easily trim their AI exposure without leaving the index entirely.
Anyone holding an S&P 500 fund alongside individual positions in Nvidia, Apple, or Microsoft may be more concentrated in AI than their account summary suggests.
Disclosure: This article was edited by Diego Almada Lopez. For more information on how we create and review content, see our Editorial Policy.