Those bills are paid with borrowed money. The volume of outstanding student loans is rising rapidly and now exceeds credit card debt, though recent reports of it crossing $1 trillion may be premature. Moody's Analytics puts the number at around $750 billion. But while credit card debt is declining, student loan debt keeps going up.

Just like housing, many student loans were made with little or no research into whether borrowers were fit. Federal Stafford loans are basically automatic for college students, and government backing for other types of loans gave other student lenders little reason to be picky.

Defaults on federal student loans jumped from 7 percent to 8.8 percent in the most recent fiscal year. That measures just recent borrowers who were already behind within two years of their first payments coming due.

Those numbers are all alarming. But putting them in context requires thinking separately about the ideas of a “student loan bubble” and an “education bubble.”

First, one thing that's important about the possible student loan bubble is that it poses much less of a threat than housing debt did to drag down the entire economy. Yes, many individual borrowers may find themselves in trouble. But total student loans probably amount to less than 10 percent of outstanding mortgages. Every single student loan could default and it still probably wouldn't match total mortgage defaults during the recent downturn. More important, unlike mortgages, Wall Street isn't knee-deep in securities composed of bundled student loans, as it was with mortgages. (It also helps that it's also harder to speculate in student loans; an investor can flip a house, but not a brain.)

The other big difference with student loans is the dominant role the federal government has assumed in the market in the last few years: it accounts for roughly 85 percent of student debt.

That matters for several reasons.

First, the government is answerable to voters and not shareholders, so it's more likely than private investors to take steps such as those announced by President Barack Obama to try to relieve student debt burdens.