At first blush, the marriage tax seems like a reasonable target. it refers to the extra tax that most working couples pay, compared with two singles earning the same income. Sally and Joe, each with taxable incomes of $25,000, pay $3,965 apiece in federal taxes. If they marry they pay $8,930. That’s $1,000 more. Unfair, unfair.

But those who deplore this inequity overlook the marriage bonus that many other couples enjoy. That’s equally unfair, if you think that the tax code shouldn’t care if you wed or not. The bill would produce smaller deficits if Congress lowered the bonus and used that money to help the unlucky couples who are penalized. But bonuses are popular, and no one wants to take them away. Here’s who are helped and who are hurt by today’s tax system:

Critics complain that marriage tax is anti-family, discouraging people from tying the knot. But people marry and unmarry for reasons remote from the IRS. No evidence exists that erasing the tax would set off a nationwide nuptial march. What the marriage tax does appear to do is discourage a second earner from working, says Anne Alstott, associate professor at the Columbia University law school. If a single woman earns a taxable income of $20,000, she’s taxed in the 15 percent bracket. If she marries a man earning $40,000, however, an amount equal to her salary will be taxed in the 28 percent bracket. She may decide that her job isn’t worth it, after toting up the cost of her taxes, lunches, work clothes, commuting and child care. Those who think that mothers should stay home with their children might see this penalty as a pro-family plus.

There’s no easy way to erase the marriage tax, because it’s not an identifiable clause in the income-tax code. It’s a mathematical byproduct of two basic principles of taxation. First, that people with higher incomes should pay higher taxes. When one income is stacked on top of another, some money is often thrown into a higher bracket. And second, that married couples with equal incomes should be taxed alike. If everyone were taxed without regard to marital status, a husband earning $50,000, with a wife at home, would pay more than a $50,000 working couple where each earns $25,000. Then you’d hear shrieks about a “homemaker” penalty. Traditionalists would stomp anyone who even suggested it.

A flat tax would solve the problem, by taxing all incomes individually and at the same rate. But it wouldn’t raise enough revenue without hiking the rates for middle-income families also a non starter today.

The bill now in Congress proposes a working-couple tax credit to offset the marriage penalty. But it could be a mess to claim. To discover the size of your credit, you’d have to do some special calculations–for yourself as single, your spouse as single and you both as a couple. Aaargh. And your benefit would be small, because Congress intends to cap each year’s cost at $2 billion. No couple could get more than $145.

A simpler solution would be one tried in the early 1980s: giving a modest tax deduction, keyed to the income of the secondary earner. But why bother doing even this, just to put $12 a month or less into working couple’s pockets? Congress has a $203 billion deficit to break. Every little $2 billion helps.

Tax laws favor one-career couples. Here’s a tally, for married without kids.

INDIVIDUAL INCOME MARRIAGE HUSBAND WIFE BONUS PENALTY $25,000 $5,000 $306 – 25,000 25,000 – 286 50,000 10,000 1,144 – 50,000 50,000 – 1,326 300,000 – 5,345 – 300,000 300,000 – 15,521