You mean low income inequality, right?
That post WWII era in which the US was pretty much the only developed nation with it's manufacturing capability still intact. That and the GI Bill explain how that era was exceptional - a large number of folks who returned from WWII got free college educations.
Minor thoughts:
a) trivial quibble: Spain was not intact in 1940, I doubt they'd recovered that much by 1945? b) I was gonna say something bout Argentina not being a developed country, but I looked it up, and TIL that Argentina used to be super developed. TIL! c) most of those undeveloped countries had virtually no manufacturing, which is why UncleOxidant excluded them in a comment about manufacturing power
But more importantly...
The numbers I've seen bandied about (I don't have credible sources) say that the US was 50% of world GDP, and held 80% of hard currency reserves, in 1945. Supposing that's correct, even though you're technically right that the US wasn't "the only developed nation ... still intact", it's still almost-correct-in-spirit. US manufacturing capability in 1945 was way way way ahead of every other country. The point being that if the US economy experiences unusual behaviour in the period 1945 to 1971, one obvious factor is its overwhelming global supremacy in manufacturing in the immediate post-war period.
The US benefits a lot from simply being big. On some level success is simply about biomass - you need the largest number of brains you can, sitting around thinking about stuff.
Fixed, thank you.
When we're unformed voters, then who we vote for and what policies end up getting enacted to not serve our best interests.
While this at first may not seem like much, it's a key butterfly effect for many of the other problems we have today. It's not the whole picture, but the root of a larger picture.
(I don't know. But it has to be asked)
The answer is that 1875-1920 looks an awful lot like 1975-2020.
(We might have recently passed Belle Epoch-levels of inequality - not sure this is an accomplishment we should brag about)
Its worth noting that large scale disasters (of all sorts) were always great equalizers, a countervailing force to rising inequality. In general, the more you had to lose, the more you lost.
We've been getting better at preventing or coping with a lot of these things like plagues, famines, and so on, but we've become even better at insulating accumulated wealth from being destroyed by them (including by financial crises), by privatizing gains and socializing losses.
I have a feeling that we've entered a new period of larger scale social unrest and natural disasters which may or may not significantly destroy and/or redistribute wealth to level the playing field again somewhat.