Moreover, we have no clue whether Mr. Romney paid any gift tax on transfers, now valued at $100 million, to a trust he set up in 1995 for the benefit of his five sons. Until this year, the federal gift tax had a lifetime exemption of $1 million, and it taxed gifts in excess of that amount at rates between 29 and 44 percent. A gift of $100 million to one’s children could, therefore, require paying a tax of as much as $29 million to $44 million.

But every good tax professional knows that gift tax returns are rarely audited, except after the transferor’s death. And normally the I.R.S. cannot challenge such a return after three years from its filing. But if the values of the gifts were not properly appraised and disclosed on Mr. Romney’s gift tax returns, a challenge may still be possible. If he did not file any gift tax return, he would still be liable for the tax, plus interest and penalties.

Based on his aggressive tax planning, revealed in the 2010 returns he has released and his approval of a notably dicey tax avoidance strategy in 1994 when he headed the audit committee of the board of Marriott International, my bet is that — if Mr. Romney filed a gift tax return for these transfers at all — he put a low or even zero value on the gifts, certainly a small fraction of the price at which he would have sold the transferred assets to an unrelated party. Otherwise, he should be happy to release his gift tax returns. According to a partner at Mr. Romney’s trustee’s law firm, valuing carried interests, such as Mr. Romney’s interests in the private equity company Bain Capital, at zero for gift tax purposes was common advice given to clients like Mr. Romney in the 1990s and early 2000s.

If detected, undervaluing large gifts to one’s children could provoke large penalties from the I.R.S. These are the kinds of tax penalties that even multinational corporations try to avoid because they fear how the public would react to the adverse publicity that would inevitably follow.

To settle these questions, Mr. Romney should release his gift tax returns, or other documents showing how he valued his transfers to his family’s trust and to his I.R.A., and at least three additional years of income tax returns.

No one should begrudge Mr. Romney or his family the wealth they have earned. But if he has not paid the taxes that apply to transfers of such wealth, this should concern us all. After all, who do you think pays for the shortfall?