JPMorgan analysts report that a basket of 41 AI-linked companies now accounts for 49% of the S&P 500 index by market capitalization. This concentration is the second-highest in the history of the index, with levels last seen during the 19th-century railroad boom.
Data from Bianco Research confirms this trend, estimating that a similar 41-stock AI basket held between 48.7% and 49% of the index as of mid-October 2026. These companies were responsible for approximately two-thirds of the total market capitalization growth of the S&P 500 this year.
Individual stock weightings highlight the concentration: Nvidia holds 8.21%, Apple 7.05%, Alphabet 6.13%, Microsoft 5.65%, and Amazon 4.02%. Furthermore, the top 10 stocks in the index now represent 38–40% of its total value, the highest level recorded since 1965.
The Information Technology sector reached a weight of approximately 39% earlier in 2026, exceeding the 33% peak observed during the dot-com era. Analysts note that the S&P 500 is increasingly bifurcated into AI-focused firms and the rest of the market.
