This is much more speculative, but perhaps within-country divergence and convergence explains more of the time-variation in national growth rates than we realise.
One of the very useful reminders of William Easterly’s Tyranny of Experts is that the nation-state is often not the appropriate unit at which to consider big questions of growth and development. For example, if we look at per capita gross world product, there is no slowdown to explain. In fact growth seems to accelerate in the final quarter of the 20th century. The growth-pessimists will respond that this is merely ‘catch-up’, countries far from the technology- and productivity- frontier moving closer to it, rather than a pushing-forward of the frontier itself.
Of course they’re correct, but if that reasoning applies to the late-20th and early-21st century world, why should it not apply within the mid-20th century United States?