If a U.S. citizen or resident alien is married to a nonresident alien, a joint return may not be filed unless both spouses file an election with the IRS to treat the nonresident alien as a resident alien.

While it may appear that a nonresident alien who has no income from U.S. sources need not be concerned with filing status, since he or she is not subject to taxation by the U.S. government, the filing of a joint return may at least reduce his or her spouse's U.S. tax bill on income subject to U.S. taxation. If the nonresident alien has U.S. source income, an election for a joint return may be doubly beneficial, since he or she may be taxed on the U.S. income anyway.

Nonresident aliens who are not treated as resident aliens generally are taxed only on their income from U.S. sources. Investment income from U.S. sources is taxed at a flat 30 percent rate. Regular U.S. income tax rates for a married person filing separately are imposed on income from a job in the U.S. This income may be offset by deductions.

In contrast, when a nonresident alien is treated as a resident alien, he or she is subject to U.S. taxation on worldwide income (i.e., income from foreign sources as well as U.S. sources). However, as with any other U.S. citizen or resident alien, the alien is eligible for a foreign tax credit or an itemized deduction for taxes paid to a foreign government on income earned in those countries. The alien may also exclude up to $85,700 of foreign earned income from U.S. taxation in 2007 and may be eligible to exclude or deduct some of the cost of foreign housing.

You can make the election to treat a nonresident alien spouse as a resident alien by attaching a statement to that effect, signed by both you and your spouse, to the joint return for the first tax year of the election. The election is effective for all later tax years unless it is revoked by the spouses or the IRS, one spouse dies, or the spouses are legally separated. It will also be suspended as of any year in which neither spouse is a U.S. citizen or a resident alien.