The 400 highest-earning households in the United States averaged nearly $345 million in 2007, up 31 percent from a year earlier, data from the Internal Revenue Service shows.

The figures for 2007, the last year of an economic expansion, show that average income reported by the top 400 earners more than doubled from $131.1 million in 2001. That year, Congress adopted tax cuts proposed by George W. Bush. Democrats say those cuts disproportionately benefit the wealthy.

Each of the top 400 earning households paid an average tax rate of 16.6 percent, the lowest since the I.R.S. began tracking the data in 1992, the statistics show. Their average effective tax rate was about half the 29.4 percent in 1993, the first year of the Clinton administration, when taxes were increased.

The statistics underscore “two long-term trends: that income at the very top has exploded and their taxes have been cut dramatically,” said Chuck Marr, director of federal tax policy at the Center on Budget and Policy Priorities, a Washington research group that supports increasing taxes on high-income people.