In a new sign of increasing inequality in the U.S., the richest 1 percent of Americans in 2006 garnered the highest share of the nation's adjusted gross income for two decades, and possibly the highest since 1929, according to Internal Revenue Service data. Meanwhile, the average tax rate of the wealthiest 1 percent fell to its lowest level in at least 18 years. The figures are from the IRS's income-statistics division and were posted on the agency's Web site last week. The 2006 data are the most recent available. According to the figures, the richest 1 percent reported 22 percent of the nation's total adjusted gross income in 2006. That is up from 21.2 percent a year earlier, and is the highest in the 19 years that the IRS has kept strictly comparable figures. The 1988 level was 15.2 percent. Earlier IRS data show the last year the share of income belonging to the top 1 percent was at such a high level as it was in 2006 was in 1929, but changes in measuring income make a precise comparison difficult. The average tax rate in 2006 for the top 1 percent, based on adjusted gross income, was 22.8 percent, ... the fifth straight year of declines. The average tax rate of this group was 28.9 percent in 1996.