Rapid run-ups in debt are the single biggest predictor of market trouble. So it is worth noting that over the past 10 years the amount of student loan debt in the U.S. has grown by 170 percent, to a whopping $1.4 trillion — more than car loans, or credit card debt. Indeed, as an expert at the Consumer Financial Protection Bureau recently pointed out to me, since 2008 we have basically swapped a housing debt bubble for a student loan bubble. No wonder New York Federal Reserve president Bill Dudley fretted last week that high levels of student debt and default are a "headwind to economic activity." Read more from The Financial Times:

Donald Trump still open to bank break-up proposals

New York Fed chief warns on US student debt

The tangled web of Gary Cohn, Goldman Sachs and Glass-Steagall In America, 44 million people have student debt. Eight million of those borrowers are in default. That's a default rate which is still higher than pre-crisis levels — unlike the default rate for mortgages, credit cards or even car loans. Rising college education costs will not help shrink those numbers. While the headline consumer price index is 2.7 percent, between 2016 and 2017 published tuition and fee prices rose by 9 percent at four-year state institutions, and 13 percent at posher private colleges.

The student loan market is hopelessly opaque — only a quarter of students can predict their own debt load.

A large chunk of the hike was due to schools hiring more administrators (who "brand build" and recruit wealthy donors) and building expensive facilities designed to lure wealthier, full-fee-paying students. This not only leads to excess borrowing on the part of universities — a number of them are caught up in dicey bond deals like the sort that sunk the city of Detroit — but higher tuition for students. The average debt load individual graduates carry is up 70 percent over the past decade, to about $34,000. Having just attended the first college preparation meeting at my daughter's high school, where I was told to expect a $72,000 a year sticker fee for Ivy League and liberal arts colleges, I would feel lucky to get away with just that. This is clearly, as Mr. Dudley observed, a headwind to stronger consumer spending. Growing student debt has been linked to everything from decreased rates of first time home ownership, to higher rental prices, to lower purchases of white goods and all the things that people buy to fill homes. Indeed, given their debt loads, I wonder how much of the "rent not buy" spending habits of millennials are a matter of choice. But there are even more worrisome links between high student debt loads and health issues like depression, and marital failures. The whole thing is compounded by the fact that a large chunk of those holding massive debt do not end up with degrees, having had to drop out from the stress of trying to study, work, and pay back massive loans at the same time. That means they will never even get the income boost that a college degree still provides — creating a snowball cycle of downward mobility in the country's most vulnerable populations.