The 400 wealthiest taxpayers accounted for more than 1 percent of all the income in the United States in the year 2000, more than double their share just eight years earlier, according to new data from the Internal Revenue Service. But their tax burden plummeted over the period.

The data, in a report that the I.R.S. released last night, shows that the average income of the 400 wealthiest taxpayers was almost $174 million in 2000. That was nearly quadruple the $46.8 million average in 1992. The minimum income to qualify for the list was $86.8 million in 2000, more than triple the minimum income of $24.4 million of the 400 wealthiest taxpayers in 1992.

While the sharp growth in incomes over that period coincided with the stock market bubble, other factors appear to account for much of the increase. A cut in capital gains tax rates in 1997 to 20 percent from 28 percent encouraged long-term holders of assets, like privately owned businesses, to sell them, and big increases in executive compensation thrust corporate chiefs into the ranks of the nation's aristocracy.

This year's tax cut reduced the capital gains rate further, to 15 percent.