What's in those tax returns that Romney won't release?

Some commentators have suggested, for example, that — like tens of thousands of other Americans who have taken advantage of an Internal Revenue Service amnesty — he might not have declared and paid taxes on his Swiss bank account. I can’t imagine that he would have engaged in such blatant tax cheating. He is far too smart for that.

Another suggestion is that in 2009 he paid income taxes significantly below the 13.9 percent he paid in 2010. This is more plausible, and potentially more damaging politically, even if perfectly legal.

After all, the one year’s tax returns that he has released raise doubt about his campaign’s claims that his offshore accounts did not save him one penny of tax. Putting business assets into an individual retirement account invested in a Cayman Islands corporation allows Mr. Romney to avoid the “unrelated business income tax” — a 35 percent levy — on at least some of his I.R.A.’s earnings, a tax that he would have had to pay if his I.R.A. were held directly by a financial institution in the United States.

With an I.R.A. account of $20 million to $101 million, the tax savings would be more than a few pennies.