AI stocks now make up 49% of the S&P 500, the second-highest concentration on record

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 approx...