(1) If one spouse files separately and itemizes deductions, then both must do so.

(2) An itemized deduction of a married taxpayer who files a separate federal income tax return claiming the itemized deduction may not be claimed as an itemized deduction by the taxpayer's spouse.

(3) If a married taxpayer files a separate Montana return, the following rules apply if the taxpayer filed a joint federal income tax return with the taxpayer's spouse or did not file a federal income tax return:

(a) an itemized deduction clearly attributable to one spouse may be claimed only by the spouse to whom it is attributable; and

(b) if the spouses file a separate return on the same form, an itemized deduction not clearly attributable to one spouse may be divided as provided on the form;

(c) if one or both of the spouses files a separate return on a separate form, an itemized deduction not clearly attributable to one spouse must be divided equally unless the spouses enter into a binding written agreement providing a different division;

(d) the agreement described in (3) (c) is a tax record each spouse must retain and provide to the department on request.

(4) If a taxpayer files a Montana return claiming an adjustment to gross income or an itemized deduction that is allowed only to taxpayers claiming a specific federal filing status, except as follows, the adjustment is disallowed unless the taxpayer files their Montana income tax return using the same status:

(a) a capital loss of up to $1,500 is allowed a married taxpayer filing separately but must be calculated using only that spouse's gains and losses; and

(b) a taxpayer who converted an IRA to a Roth IRA and elected to defer the income ratably may file separately and defer the gain.