The 2024 forecast is particularly pessimistic about China’s fertility prospects. Both projections produce very substantial global aging, a major global capital glut producing very low long-run real capital returns. The latest forecast entails 10% lower global GDP in 2100 and far higher payroll tax rates to fund old-age benefits. Most important, it entails a major change in the course of economic hegemony with China’s 2100 global GDP share falling from 25.6% to 14.9% and the US share rising from 11.2% to 14.4%. Our results are sensitive. Should the US eliminate all future immigration, its 14.4% global 2100 GDP share would drop to 9.2%. And were global fertility to follow the UN’s low variant, 2100 world output would be one third, not one tenth lower. The level and division of global output is also highly sensitive to the speed at which AI expands frontier technologies. Accelerated AU/AI – 4x faster-than-recent growth in capital’s share through 2050 – or Transformative AU/AI – 10x faster capital-share growth – reinforce demographic forces, ensuring long-run US economic hegemony. Indeed, Transformative AI combined with 2024 demographics implies US and Chinese 2100 global GDP shares of 25.3% and 16.9%, respectively.

That is from a new NBER working paper by Seth G. Benzell, Laurence J. Kotlikoff & Victor Yifan Ye.  Note that today the U.S. share of global gdp is slightly higher than it was in 1980.

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