When their mortgages went underwater, Americans stopped buying, and the rest of the economy, which had been driven by consumer spending, sputtered, too. “Households, especially at the bottom, are very sensitive to house prices,” he said. “In 2008 and 2009, they drove the recession. We basically missed the boat.”

Mr. Sufi’s thesis helps to explain one of the mysteries of the U.S. economy today. As a report published this month by Bain and Company explains, money is cheap and plentiful. Titled “A World Awash in Money,” the Bain analysis argues that one of the defining realities of this decade will be a global “capital glut.” Bain predicts the global capital pool will swell to nearly $1 quadrillion by the end of this decade, creating an ocean of “capital superabundance” 10 times greater than the world’s underlying gross domestic product.

Here’s the puzzle: With so much money sloshing around, why is the recovery so lackluster? In Mr. Sufi’s view, one reason is that not everyone is able to dip into this overflowing capital pool. As he explained in a recent paper: “It could be that there’s a 55-year-old household that had their house price drop by 40 percent and they therefore have no net worth available to them. They were planning on drawing down home equity for retirement, and they find themselves underwater, and they cut back on consumption and they save like crazy. If this describes the world, policy could support the banks with infinite resources and it’s not going to lead to any additional spending by levered households.”

Income inequality is central to this story: The financial crisis has hit poorest households hardest. Mr. Sufi has found that the households in the 90th percentile of net worth distribution have seen almost no decline in their wealth. Meanwhile, households in the middle and at the 25th percentile have experienced “a dramatic reduction in net worth.” That’s because financial assets, which have rebounded strongly, are overwhelmingly owned by households in the 90th percentile. If households in the median and at the 25th percentile have any assets at all, “it’s basically housing assets.”

As Mr. Sufi points out, “given the strong recovery in financial asset prices, but the languishing housing market, we can see why the lower part of the net worth distribution has been hammered in this recession.” The result is constrained consumer spending and a sputtering recovery.