Earlier quoted context omitted.
I also wondered the same thing and sought out data on it a while back. I provided those data in this thread here [1]. The gist of it is that our hypothesis does seem to be supported by the data. And I think it makes sense if you consider things in a thought experiment. In a nation with a king and a 100 impoverished citizens inequality would be mostly invisible. But now imagine our king decides he's been cruel keeping…
Most people are not interested in being as wealth as the king, or the nobles, but they are interested in having an extra pig and a hut that doesn't leak.
But of course people are not suddenly content. When the standard of living changes people take it for granted and then seek more. It's not a bad thing. This never-ending desire for more is undoubtedly a big part of the evolutionary drive that's brought us to where we are, but it also means people will never be content. Describe the life of the poor today to people from not all that long ago and it would seem like we live in a utopia - air conditioning, electric toys, color television, cars, regular access to food, indoor plumbing? But of course we see things quite differently! And I expect in a few hundred years we'd see the lives of the poor of that time as something like utopia, but they too will undoubtedly see things quite differently! Man's nature is a great and awful thing!