I’d sort of intuitively remembered experiencing this, perhaps from my own observations while moving across the country as a kid more than most military brats:
Today, the story of America is largely the story of two economies — rural and urban. It was not always this way. The antitrust movement of the 1940s not only targeted giant firms, but was also an attempt to weaken regional centers that had amassed too much power. This largely worked and, by the mid 1970s, there was a fairly uniform American standard of living — being middle class in the Mideast was pretty much the same as middle class in New England. However, in the 1980s, many of the policies that helped ensure this balance between regions was neglected or reversed.
Emphasis mine. I’d not seen that concept (rising inequality, yadda yadda) stated quite that way before.