Contemporary American society is, in a sense, a split-level structure. Its political and social institutions provide universally distributed rights and privileges that proclaim the equality of all citizens. But its economic institutions rely on market-determined incomes that generate substantial disparities among citizens in living standards and material welfare. The differentials in income are meant to serve as incentives -- rewards and penalties -- to promote efficiency in the se of resources and to generate a great, and growing, national output.
I'm always struck by passages like this that still seem relevant, this one from this 1975 Brookings institute book.