26 U.S.C. § 6663 : US Code - Section 6663: Imposition of fraud penalty

    (a) Imposition of penalty
      If any part of any underpayment of tax required to be shown on a
    return is due to fraud, there shall be added to the tax an amount
    equal to 75 percent of the portion of the underpayment which is
    attributable to fraud.
    (b) Determination of portion attributable to fraud
      If the Secretary establishes that any portion of an underpayment
    is attributable to fraud, the entire underpayment shall be treated
    as attributable to fraud, except with respect to any portion of the
    underpayment which the taxpayer establishes (by a preponderance of
    the evidence) is not attributable to fraud.
    (c) Special rule for joint returns
      In the case of a joint return, this section shall not apply with
    respect to a spouse unless some part of the underpayment is due to
    the fraud of such spouse.