veryone knows that couples who might be prosperous on a single income can, by virtue of a second, live really, really well. That second income, even if relatively modest, can pay for fancy vacations, lots of restaurant dining and maybe even luxurious automobiles. That's obvious, right?

Well, no. If the second income is that of a neurosurgeon, great. But if, as is often the case, the second income is a lot smaller than the first, the sad truth is that it hardly makes any difference at all. Perhaps it makes some difference, to the extent that it lulls two people into spending more because they work so hard and, well, they make so much money. Take into account a host of job-related expenses, and second incomes can actually cost more than they produce.

The main culprits are a tax system that savagely penalizes second incomes and the high cost of quality child care. The result, among financially savvy couples with a single high wage-earner, is that spouses (usually women) with much less earning power often stop working until the kids are in school. Those who stay on the job do so not for the money, but for the challenge and fulfillment they derive from work outside the home.

Of course, most married women work because their income really does make a big difference in their family's standard of living. Indeed, many work just to put food on the table. But for affluent couples whose partners have seriously unequal earning power, the cost of that second income often means the lower-paid partner is working for free.

Debra Gendel, for instance, was a fashion editor married to a successful Los Angeles television executive. When she took a good hard look at the taxes she was paying, the cost of commuting, dry cleaning and child care, it began to dawn that her glamorous, well-paid job just wasn't paying well enough. She quit to stay home with her children.

"You really have to love your work and think you're doing a greater good for mankind," she says, acknowledging that she is fortunate to have the option to stay at home. She adds that her decision gave her husband's career a boost because he's now free to work longer and not worry about home matters that she can attend to. This is no mere rationale; there is evidence that executives with stay-at-home wives on average earn more than their colleagues in two-career households.

Gendel's case is far from isolated. Peggy Ruhlin, a financial planner and certified public accountant in Columbus, Ohio, has found it necessary to break some hard truths about second incomes to her clients. She cites the case of a highly-paid executive whose wife worked for a government social work agency. Her position was consuming and paid less than $25,000 a year, but it was deeply satisfying because she was helping people in need. Ruhlin ran through the numbers and showed that the wife was taking home a grand total of $1,500 a year when all was said and done.

How is that possible? First, her income pushed the couple from the 31percent federal tax bracket into the 36 percent bracket. Add 8 percent in state and local taxes (although these are federally deductible) and nearly 8 percent of the woman's income for Social Security and Medicare taxes. The result? Taxes alone consumed half her earnings.

Ruhlin says the Social Security cut was especially unkind because, on retirement, the wife will be entitled to the equivalent of half her husband's entitlement (he'll still get the full amount) even if she never worked at all. Her contributions from a relatively low wage job would never entitle her to more on her own, and so her payments will never do her any good.

On top of all this were child care, commuting and other expenses. When the planner broke the news, the woman became teary-eyed. She is now considering volunteer work with more flexible hours.

Many other women in similar situations choose to remain on the job, and more men are finding themselves in that spot, and making that choice, as well.

"It's a lifestyle decision," says Ronald Roge, a financial planner in Centereach, N.Y., who has often encountered the situation. "That's the softer side of it. Some people are just tired of staying home, or they get fulfillment from going to work."

If you have children and a well-paid spouse, consider the costs of any job before you take one. This is not to say you shouldn't work. But why take a job unless you know what it really pays? When you discover that it pays nothing, you might decide to take a different one that is more fun or rewarding. Or you might decide not to take one at all.

"It's risky to automatically assume another income will balance the books or provide the wherewithal for a better life," writes Linda Kelley in Two Incomes and Still Broke? (Times Books, 1996). "For most families there are monetary benefits, although not usually as many as expected, if both partners work outside the home. For others, the effort is wasted in a flow of job expenses that devour the second income and leave nothing but a bewildered and angry couple arguing over where the extra money went."

Kelley and financial planners such as Ruhlin cite many obvious and not so obvious costs. Perhaps foremost among these is taxes. If you earn $35,000 and your spouse earns $100,000, you'll be taxed at the rate normally charged to someone making $135,000. And you'll have to pay Social Security taxes as well. In states with hefty income taxes (such as New York and California), this can mean that 40 percent to 50 percent of the second income goes to taxes.

"The United States tax system is a product of the 1930s and 1940s," says Edward J. McCaffery, a University of Southern California law professor whose new book, Taxing Women (University of Chicago Press) argues that the tax system is biased against working women. "At that time the single-earner model was the norm for families - men worked outside the home and women worked inside it. Tax policy decisions favored and rewarded this arrangement and made it difficult to be a two-earner family." He adds that, "Over time those biases have gotten worse."

But taxes are only the beginning of the story. Other major costs include:

If you plan on putting your kids in preschool anyway, don't count this. But if you need day care to work, deduct this from your earnings. The cost can range from $4,000 to $25,000 a year, and can easily wipe out whatever's left of a second income after taxes. Sue Sharp, a singer and voice teacher who has two children, says she gave up teaching when she realized she was "working just to pay for day care. It just was not worth it."

Figure on about 30 cents a mile if you drive. That's $1,125 a year if your round-trip is 15 miles. And if you're figuring your hourly earnings, add in your time on the road.

Many people eat breakfast out as well, and buy several cups of coffee or snacks during the day. At $6 daily (and you probably spend more), this costs $1,500 a year.

Working couples probably eat out more, which can cost plenty compared to eating at home. Harried working couples are probably also likelier to give the kids money for lunch rather than a homemade sandwich. Put it down for another $1,000 or more.

Business attire, dry cleaning and even the right kind of car fall into this category. It can run from $250 on up into many thousands. As a fashion editor, for example, Gendel needed a chic wardrobe that cost plenty. And if you're in sales or otherwise visit clients, driving up in a jalopy gives the wrong impression.

Kelley says working couples, induced by guilt, often buy unnecessary things for their kids because they get so little time with them. Also, busy couples often don't have time to comparison shop. They frequently buy expensive takeout food, or groceries from a high-priced convenience store instead of a cheaper supermarket. All these costly habits can be considered job-related expenses.

The really bad news about such major job-related expenses as taxes and child care is that, usually, you can't do much about them. On the other hand, many work-related costs can be reduced.

Bringing lunch to work can save a bundle, for instance. Self-employed people can use Keogh retirement accounts to shelter income from taxes, but on the other hand, the self-employed pay much higher Social Security taxes. Roge says spouses who work together in their own business can do themselves a big favor by paying one spouse only a nominal amount and funneling all the money through the other. It's legal, he says, and results in substantially lower taxes, since the optimal tax situation is to have a single, high-income earner.

Also on the bright side, discovering that a second income really doesn't contribute much financially can free a spouse to learn new skills, go back to school, pursue an unprofitable dream career, or launch a home-based business that may pay little during the first few years. If you're going to work for free, after all, you might as well enjoy it.