(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.