In 2007 the top 400 taxpayers had an average income of $344.8 million, up 31 percent from their average $263.3 million income in 2006, according to figures in a report that the IRS posted to its Web site without announcement that were discovered February 16.

Their effective income tax rate fell to 16.62 percent, down more than half a percentage point from 17.17 percent in 2006, the new data show. That rate is lower than the typical effective income tax rate paid by Americans with incomes in the low six figures, which is what each taxpayer in the top group earned in the first three hours of 2007.