It so happens that this summer the Internal Revenue Service released data from the 400 individual income tax returns reporting the highest adjusted gross income. This elite ultrarich group earned on average $202 million in 2009, the latest year available. And buried in the data is the startling disclosure that six of the 400 paid no federal income tax.

The I.R.S. has never before disclosed that last fact.

Not even Mr. Romney, with reported 2010 income of $21.7 million, qualifies for membership in this select group of 400. But the data provides a window into the financial lives and tax rates of the superrich. Since the I.R.S. doesn’t release data for the tiny percentage of Americans at Mr. Romney’s income level, the 400 are the closest proxy.