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.