In contrast to the axiom about acquiring yachts - if you have to ask what it costs, you can't afford it - a new population study suggests that if you have children, you probably didn't ask beforehand if you could afford them.

Because if you had, the chances are you would have found out that you couldn't.

"The Value and Cost of Children," published in the April bulletin of the Population Reference Bureau, Inc., estimates that if your family income is from $16,500 to $20,000 after taxes, it will cost you $53,605 to get a child to see him or her through college.

For families living at a lower level, with incomes from $10,500 to $13,500 after taxes, the estimate is $35,261 to age 18, and $44,156 through college.

This is assuming that the moderate income child attends a four-year public university, and the lower-income child a four-year public institution other than a university. Never mind private schools.

And it is also based on current costs. The report quotes a study by the Oakland Financial Group of Charlottesville, Va., which calculates that in the 1990s, which is college time for a child now a year old, four years at a state university will cost $47,330; and at a private one, it will be $82,830.

But is it worth it?

This study, by Florida State University Prof. Thomas J. Espenshade, kept asking that queston and kept getting unclear answers. He quotes other researchers as having reported that "Most parents appeared to have little notion of what it actually costs to raise a child, and many were reluctant to discuss children as if they were consumer goods."

The answer may be that if you have to ask what the children are worth, you don't want any. But Espanshade suggests in his conclusion that telling people the cost might be an effective means of birth control.

"As a means of reducing fertility this proposal takes on added cogency, since there is some evidence to indicate that parents vastly underestimate the expenses involved in childrearing," he writes.

However, one popular parents' misconception about children and money is already disappearing as societies modernize. That is the idea that you're ever going to see a financial return on that crop of children.

"There is no evidence in the United States that children are raised for profit" he quotes other researchers. But, Espenshade added, "In lesser developed countries, parents are under the impression that children are a net economic surplus."

It's a mistake, he said. Not only does the return amount to a lot less than the investment, but there is "a growing risk factor" in traditional societies, as children get away from the idea that they are obligated to support their parents in old age.

Espenshade believes that if such people got the true picture, and had some other form of old-age protection, such as Social Security, they would have fewer children.

Caucasian families studied in Hawaii didn't even expect the kids to pitch in around the house, let alone insure their old age. They "wanted girls both for the companionship they would supply for the mother and for the congenial personality traits they were perceived to possess. Boys, on the other hand, were wanted primarily to continue the family name mentioned as a reason by almost half of respondents) and also to furnish companionship for the father.

"Little practical help with household chores was anticipated from either sons or daughters."

Non-economic reasons given for having children were: proof of one's characteristics, affording parents "the opporunity to sacrifice for the good of someone else," establishing the emotional security of family life, creativity and accomplishment in childrearing, enabling "parents to influence the course of others' lives," prestige, and a category called "stimulation, novelty and fun," which was explained by the idea that "a birth creates the sense that something new and different is happening, and in so doing may help to relieve the tedium of everyday life."

When you add up all that, then you have to decide whether it's worth it. That's the $64,215 question.