New car loans have become the new hot product and Wall Street, not car-loving Americans, is the real market. The growth of student loan debt has received no shortage of attention from politicians and the media in recent years, making it one of the top economic anxieties in post-mortgage-meltdown America. Behind the front-page investigations and populist political platforms, the numbers are indisputably chilling: student loan balances have surged from less than $18,000 per person to $25,000 per person in the ten years since 2005. But another class of debt is also growing at troubling rates without attracting anywhere near the...