How much of the stock market is propped up by the AI boom?

A review found 47 AI-related stocks accounted for about 60% of S&P 500 gains, and roughly 75% in the first half of 2026. The 10 largest firms hold about 40% of the index's value, while Amazon, Microsoft, Google and Meta are set to spend over $720B on data centers and capex this year, prompting investor questions about returns.

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