A new analysis of data from the Internal Revenue Service shows income inequality grew significantly in 2005, creating the biggest earnings gap between the wealthy and middle class since 1928.

While the nation's total income increased 9% that year, virtually all of that growth was enjoyed by the 10% of Americans making $100,000 or more.

Average incomes for those in the bottom 90% actually fell 0.6% from 2004.

As a result, the 300,000 top-earning Americans collectively made nearly as much as the 150 million lowest-earning Americans.

The study was conducted by economists Emmanuel Saez at the University of California, Berkeley, and Thomas Piketty of the Paris School of Economics.

"If the economy is growing but only a few are enjoying the benefits, it goes to our sense of fairness," Saez told the New York Times. "It can have important political consequences."