As college students head back to school, the more sensible of them are wondering how they will pay for the privilege.

There are some 21.5 million students in the United States this year, and many of them will be funding their college careers on borrowed money. Given that there is at present more than $1.3 trillion in student loans on the books, it's pretty clear that many college students are far from sensible.

The average student's debt upon graduation now approaches $40,000, and as college becomes ever more common and expensive, calls to make it "free" seem to be multiplying. Even Hillary Clinton says that when it comes to college, "Costs won't be a barrier."

But the only way college could be free is if the faculty and staff donated their time, the buildings required no maintenance, the campuses required no heating, cooling, or utilities of any kind, and every other expense simply disappeared. As long as it is impossible to produce something from nothing, though, costs are absolutely a barrier.

The actual question we debate, regardless of whether we admit it, is who should pay for people to go to college.

If students do not shoulder the cost, that cost will be shifted to someone else. And this is where things get interesting. No one seems to take the idea of free college for everyone seriously, but the idea of student loan forgiveness has somehow gained traction. In the end, though, it amounts to the same thing:

The American taxpayer will be left holding a very expensive bag.

But if taxpayers are to bear the cost of student loan forgiveness, shouldn't they have a say in how their money is used?

At a bare minimum, taxpayers should be able to decide what students will study on the public dime. If we are going to force taxpayers to foot the bill for college degrees, students should only study those subjects that are of greatest benefit to the taxpayers. After all, students making their own choices in this respect is what gave us the student loan problem in the first place. We simply do not need more poetry, gender studies, or sociology majors. Starbucks is fully stocked with baristas for the foreseeable future.

How do we know which subjects benefit society? Easy.

Average starting salaries give a clear indication of what type of training society needs its new workers to have. Certainly, there are benefits to a college major beyond the job a student can perform. But if we're talking about the benefits to society - as opposed to the benefits to the student - the only thing that matters is what the major enables the student to produce for society. And the value of what the student can produce is reflected in the wage employers are willing to pay the student to produce it.

Today's wages tell us that we could use more computer engineers (average starting salary $65,000), nurses ($55,000), and statisticians ($53,000), and far fewer criminal justice majors ($35,000), social workers ($33,000), and elementary education majors ($32,000).

A low wage does not mean, for example, that elementary education isn't important. It is. A low wage simply means that there are already so many people in the market trained to teach elementary school that we don't need more of them.

Meanwhile, there are few who are willing and able to perform jobs requiring a petroleum engineering major, so the value of one more of those people is very high (average starting salary, $103,000).

So we can go this way. We can have taxpayers pick up students' tuition in exchange for dictating what those students will study. Or we can allow students both to choose their majors and pay for their educations themselves. But in the end, one of two things is true:

Either a college major is worth its cost or it isn't. If the major is worth its cost, taxpayer financing isn't needed. If the major is not worth its cost, taxpayer financing isn't desirable. Either way, taxpayers have no business paying for students' educations.

Antony Davies is associate professor of economics at Duquesne University in Pittsburgh.

James R. Harrigan is director of academic programs at Strata in Logan, Utah.