If you're married, you and your spouse must decide whether to file jointly or separately.

In the vast majority of cases, filing jointly will result in a lower total family tax bill. The tax rates for joint filers have the effect of "leveling out" the income, exemptions, and deductions of each spouse, so that each spouse is essentially treated as having half of the total net income. For slightly more than half of married couples, this results in a "marriage bonus," because the couple pays less in tax than they would if they each made the same income and were single. This is usually the case where one spouse's earnings are much higher than the other's.

For couples where each spouse makes roughly the same income, however, there is often a "marriage penalty" in which the second wage earner's income pushes the couple into a higher tax bracket than either spouse would face if they were single. This situation can sometimes, but unfortunately not often, be helped if the couple files separately rather than jointly.

However, the IRS does not smile on couples who file separately, perhaps because it has to incur the extra charges of processing two tax returns rather than only one.

• Under a few sets of circumstances, joint returns may be prohibited.
• In order to strongly encourage married couples to file jointly, Congress has provided that for people who are treated as "married," certain tax benefits are available only if a joint return is filed (but note that certain legally separated couples or spouses living apart from each other are not treated as married for this purpose). When you consider the effects of not being able to claim any of these benefits, it's usually clear that filing separately would result in a larger tax bill.