Elimination of the "marriage penalty," which forces some couples to pay higher taxes than they would if they simply lived together and filed as individuals, is a hot topic on Capitol Hill. But no one is talking about abolishing its flip side, the marriage bonus.

The marriage penalty affects 21 million two-income families - a sub-stantial number indeed. But 29 million families with either one income or with one spouse who greatly outearns the other get a bonus from the current tax structure.J.D. Foster, executive director of the Tax Foundation, a nonpartisan research group, said it is curious that groups now portraying the marriage penalty as unfair have little or nothing to say about the marriage bonus.

"If the pro-family groups were fighting for a fairer tax code, they would address the marriage bonus," he said in a statement to the Ways and Means Committee recently. "To my knowledge, they are silent on the bonus, so they stand self-indicted as purely special interests."

"We feel the American taxpayers are taxed enough, or too much, even those that get the so-called marriage bonus," said Bill Himpler, legislative director for Jerry Weller, R-Ill., co-sponsor of a bill to eliminate the marriage penalty.

Weller and co-sponsor David M. McIntosh, R-Ind., and conservative groups such as the Christian Coalition, argue that the marriage bonus benefits society.

"The genesis of the marriage bonus is that it rewards one-income families, where one parent stays home to raise their children," said Chris Jones, McIntosh's deputy chief of staff.

Neither the marriage penalty nor the marriage bonus is specifically intended in the tax code. Rather they are the result of provisions that determine filing status, tax brackets and standard deductions.

For instance, a one-income married couple earning $75,000 a year would pay about $4,000 less in taxes than someone who earned the same amount but filed as a single individual. The reason: The standard deductions for married couples are somewhat higher and more income can be taxed at a lower rate.

But since the deductions and tax bracket ceilings for married filers are less than double the amounts for single filers, many two-income families pay more in taxes than they would filing individual returns.

Some of the bills to eliminate the marriage penalty would allow married couples to file either as individuals or as a couple, depending on which approach taxes them least.

For Congress, the difficulty lies in trying to find a system that balances the conflicting interests of one- or two-income married couples as well as the interests of single people.

CBO says the cost of the marriage penalty to taxpayers ($29 billion) is less than the marriage bonus savings to other taxpayers ($33 billion).