Are student loans the new subprime mortgages? Among professional skeptics, the comparison has become something of a cliché, and in last weekend’s front page story, entitled “A Generation Hobbled by College Debt,” the New York Times invoked it in recounting the nightmare that student loans are becoming for so many. At the same time, others have pointed out important differences between the two kinds of debt. But history never repeats itself exactly – and there are reasons to fear that the growing mountain of student debt could have every bit as profound an impact on our economy as the housing bubble did.

Start with the structure of the student loan market. Of the roughly $1 trillion in student debt outstanding, according to a recent estimate by the Consumer Financial Protection Bureau, $848 billion consists of federal student loans, like Stafford, PLUS and Perkins loans – meaning they are explicitly backed by the U.S. government, aka taxpayers. The rest are so-called private loans, meaning they’re made by private lenders without government backing; students usually turn to these more expensive loans when they’ve exhausted other alternatives, just as homebuyers turned to subprime mortgages when they couldn’t qualify for more conventional loans .

The involvement of the government in student lending is both important and scary, because the government backing removes a level of discipline. It’s doubtful that private lenders who had to evaluate and bear the credit risk of students would extend this much money. Of course, that was also true in the housing market, where the presumed (and, as it turned out, actual) government backstop of Fannie Mae and Freddie Mac allowed debt to proliferate.

Right now, that roughly $1 trillion in student loans outstanding is paltry compared with the amount of mortgage debt outstanding at the peak of the bubble, which was about $10 trillion. Indeed, at the peak, there were about $2.5 trillion in securities backed by subprime mortgages alone, says Barron’s. And while student loans outstanding have grown rapidly – debt is up ninefold from 1997, according to the College Board’s 2011 “Trends in College Pricing” – that too is small compared with the torrid growth in subprime lending before the collapse.

But taking solace in the face value of the numbers is probably a mistake, just as it was a mistake to look at the size of the subprime market and say the problem was “contained.” (Hello, Ben Bernanke and Hank Paulson!) For one thing, just because the student loan bubble is smaller doesn’t mean there isn’t a bubble. While some of the increase in the overall level of debt has happened because more people are going to college, tuition is growing far more rapidly than inflation or even healthcare spending; in fact, according to Barron’s, tuition and fees at four-year schools grew by 300 percent from 1990 through 2011. Over the same period, broad inflation increased just 75 percent and healthcare costs rose 150 percent. Perhaps more important, education inflation has also exceeded wage growth for decades. Former U.S. Secretary of Education Bill Bennett says that tuition has increased 400 percent in the last 20 years. So by definition, an education is becoming less and less affordable – just as homeownership became less and less affordable as that bubble reached its apex.