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NewsMaterial2 min readDisconfirmAI Editorial

US Stock Market Macro Sensitivity Fracture: Independent Research Attributes Shift to Economic Regime Change, Not AI Concentration

Independent researcher Mohammad Bagher Karimi used weekly data from 2010 to 2026 to find three structural breaks in the sensitivity of US stock markets to macro factors such as real yields and breakeven inflation rates. However, changes in index weights contributed very little, and AI intensity could not explain the beta changes of top companies after controlling for size—evidence points to a shift in the macroeconomic regime.

2026-11-13 19:49 GMT+8

In a paper published in November 2026, independent researcher Mohammad Bagher Karimi examined a specific question: Has the sensitivity of the US stock market to real yields, breakeven inflation rates, oil, gold, and the dollar fundamentally changed as the S&P 500 has become increasingly concentrated in a few large technology firms? Using weekly data from 2010 to 2026 and an index reconstructed from public sources, he systematically tested this hypothesis.

Karimi identified three structural breaks, occurring respectively in early 2013, early 2020, and mid-2023. The confidence interval of the last break includes late 2022, but no break was estimated at the time of ChatGPT's release. This temporal distribution poses an initial challenge to the AI concentration hypothesis: if AI were the primary cause, the breaks should be more closely clustered around the release and diffusion of large language models.

More critical evidence comes from his precise decomposition of index beta coefficients. Karimi decomposed the change in the overall index's sensitivity to macro factors into two sources: the mechanical effect brought about by changes in index weights, and the changes in the betas of individual constituent stocks. The decomposition results showed that the contribution of weight changes was minimal. The vast majority of sensitivity changes were experienced jointly by nearly all companies, rather than being a phenomenon unique to top holdings. Specifically, after 2022, the entire market lost its positive exposure to breakeven inflation rates, which is a universal phenomenon rather than a characteristic of a few tech stocks.

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The real-yield betas of large companies did rise relative to average stocks after 2022, but Karimi further tested the AI intensity indicators in 10-K filings prior to 2022 and found that they had no additional explanatory power after controlling for firm size. In other words, there is no observable link between whether a company heavily uses AI and changes in its macro sensitivity that goes beyond the effects of scale.

Karimi's conclusion is that the evidence points to a macroeconomic regime shift, rather than concentration or AI, as the main source of the change in sensitivity. This research has its boundaries: it relies on an index constructed by a single researcher and specific weekly estimation windows, so the exact timing of the three breaks may be influenced by the sample period and estimation methods. 'Regime shift' itself is a descriptive label—Karimi's evidence indicates that the change was not driven by concentration, but he did not define what specific macro mechanisms drove this shift. For readers, the value of this paper lies in using decomposition methods to rule out concentration as the primary explanation, thereby redirecting attention back to macroeconomic conditions themselves.

Sources

  • capitalspectator.com↗
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