Wal-Mart Rules!
"Surprisingly, the primary source of the productivity gains of 1995 to 1999 was not increased demand resulting from the stock market bubble, as some economists have claimed. Nor was information technology the source, though companies accelerated the pace of their I.T. investments during those years," reports a summary of the findings published in The McKinsey Quarterly. "Rather, managerial and technological innovations in only six highly competitive industries -- wholesale trade, retail trade, securities, semiconductors, computer manufacturing and telecommunications -- were the most important causes."
https://www.mckinseyquarterly.com/Whats_right_with_the_US_economy_1151
https://www.mckinseyquarterly.com/Retail_The_Wal-Mart_effect_1152
Another groovy link. Read the executive summary at least.
http://www.ihsglobalinsight.com/publicDownload/genericContent/11-03-05_walmart.pdf
Somewhat related:
http://mercatus.org/publication/millionaires-unlikely-stay-millionaires-long
My guess is that over the last 30 years those at the mid-higher income levels have increased their worked hours by a greater amount than those in the lower income levels. (Most are salaried too!)