This post on TPMCafe and the New York times article it is based on both neatly manage to miss the likely most important cause of our growing income inequality and our lowered average incomes: the immigration of poor people to the U.S.
Between 2000 and 2005, lots of new immigrants came to the U.S., disproportionately those of lower incomes. This is going to skew the statistics on average income even if no one lost any income.
To do a study like this fairly, either everyone who immigrated to our emigrated from the U.S. during those time periods needs to be removed from the statistics, or their incomes in the other country need to be included.
That is all.