S&P 500 now 45% AI-driven as megacap tech dominance raises market risks

The article says AI-related stocks have come to dominate the S&P 500, with AI, data-center, semiconductor and energy names accounting for nearly 45% of the index’s market cap. A handful of megacaps — led by Nvidia — now drive much of index performance, raising concentration risk: Nvidia alone had a 7% weight as of March 30, 2026. Goldman Sachs and others estimate AI infrastructure spending will materially boost S&P 500 earnings (roughly 40% of 2026 earnings growth), with hyperscalers and the “Big Four” set to spend roughly $645–700 billion on AI infrastructure in 2026. The piece warns that the index has become brittle — a pullback in a few mega-caps or a slowdown in AI CapEx could trigger broad repricing — and urges focusing on durable AI adopters and infrastructure plays.