S&P 500 Bull Market Turns 4 Years Old; AI Concentration Rivals Dot-Com Bubble Levels

What’s happening

The S&P 500 bull market reached its fourth anniversary on October 12, having climbed nearly 120% since its October 2022 low. However, AI-related companies now control a share of the index that rivals the concentration seen during the dot-com bubble, raising structural vulnerability concerns.

Market impact

Elevated concentration in mega-cap AI stocks (Nvidia, Microsoft, Broadcom, etc.) means index-level gains mask significant dispersion. A rotation out of AI or a correction in the "Magnificent Seven" could trigger sharp declines in the S&P 500 despite broad-market resilience. Rising Treasury yields pose particular risk to unprofitable AI plays.

Broader implications

Bull markets do not die of old age, but they do die of excess. The current concentration mirrors 1999-2000, suggesting the market has priced in AI adoption at an optimistic pace. Valuation compression or earnings disappointment in mega-cap tech could trigger a significant correction, though breadth weakness has already begun.