Buy an S&P 500 index fund, and you'd expect to own a broad slice of American business. Yet in early October, by one count from Bianco Research, 41 artificial intelligence (AI)-related companies accounted for nearly 49% of the index's combined market value. By Bianco's reckoning, that's a higher concentration than in any past market episode except the 19th-century railroad boom. That weight means trouble in AI-linked stocks can quickly drag down the whole index, as it did last week. Only 4 of the S&P 500's 11 sectors fell, yet the index still lost nearly 2%, its worst week since June. Technology, its largest sector, dropped almost 8% and pulled it down. Many AI-linked companies are growing quickly, and their shares can swing hard on inflation data and earnings reports. Because those companies now carry so much of the index, those swings can reach almost any stock portfolio.
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