Planet Money has an interesting post regarding income inequality. From the 1930’s through the 1970’s only the bottom 90% of wage earners saw their incomes rise. High marginal tax rates certainly contributed to possible lack of increase in the high wage earners.
Something interested happened in the 1980’s and beyond: Only the top 10% of income earners have seen an increase in income. We can go back and forth about what has caused/contributed to this. That is not the question.
The question I have is how sustainable is this? How much longer can the majority of people in America not see any income/social mobility upward before civil society starts to unravel. It is not hard, or deleterious to the top 10% for the bottom 90% to have income mobility. They will not starve. They will still have more wealth than they can use for whatever purpose they want. They will not go hungry, or homeless.