All large complex human societies tend toward great wealth and income disparities (stratification). There are no significant exceptions to this rule over 6,000 years of human history. Socialist experiments in the 20th century also resulted in societies divided into the Haves and the Have-Nots.
This is a follow-up to Human Nature — Exhibit A.
The rise of a prosperous middle class in the United States was a short-lived experiment which has now reverted back to the historical norm, a kind of social regression toward the mean.
From Dave Leonhardt's A Slowdown in Growth, an Increase in Income Inequality
The first graph doesn't bother to show the change in income for Americans below the 50th percentile. This graph does.
It is remarkable that few Americans notice or seriously consider what has happened to them. Occupy Wall Street, which used "We Are The 99%" as its theme, was a small, short-lived movement which put income and wealth inequality in the spotlight in 2011. That discussion has subsequently disappeared (mostly).
Any serious discussion of the United States needs to take this blindness, or indifference, into account. It is hard for us to remember in 2012 the great struggle workers went through to raise their wages and living standards in the late 19th and early 20th centuries. Those efforts bore fruit after the Great Depression and World War II, but that's all over now.