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

A 41-company AI basket tracked by JPMorgan now accounts for nearly half the index, a level last approached by railroads in the 19th century 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. 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.

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