So how important is equality? According to the study, making an economy’s income distribution 10 percent more equitable prolongs its typical growth spell by 50 percent. In one case study, Berg looked at Latin America, which is historically much more economically stratified than emerging Asia and also has shorter periods of growth. He found that closing half of the inequality gap between Latin America and Asia would more than double the expected length of Latin America’s growth spells. Increasing income inequality has the opposite effect: “We find that more inequality lowers growth,” Berg says.

Berg and Ostry aren’t the first economists to suggest that income inequality can torpedo the economy. Marriner Eccles, the Depression-era chairman of the Federal Reserve (and an architect of the New Deal), blamed the Great Crash on the nation’s wealth gap. “A giant suction pump had by 1929-1930 drawn into a few hands an increasing portion of currently produced wealth,” Eccles recalled in his memoirs. “In consequence, as in a poker game where the chips were concentrated in fewer and fewer hands, the other fellows could stay in the game only by borrowing. When the credit ran out, the game stopped.”

Many economists believe a similar process has unfolded over the past decade. Median wages grew too little over the past 30 years to drive the kind of spending necessary to sustain the consumer economy. Instead, increasingly exotic forms of credit filled the gap, as the wealthy offered the middle class alluring credit card deals and variable-interest subprime loans. This allowed rich investors to keep making money and everyone else to feel like they were keeping up—until the whole system imploded.

THE 5 percent of Americans with the highest incomes now account for 37 percent of all consumer purchases, according to the latest research from Moody’s Analytics. That should come as no surprise. Our society has become more and more unequal.

When so much income goes to the top, the middle class doesn’t have enough purchasing power to keep the economy going without sinking ever more deeply into debt — which, as we’ve seen, ends badly. An economy so dependent on the spending of a few is also prone to great booms and busts. The rich splurge and speculate when their savings are doing well. But when the values of their assets tumble, they pull back. That can lead to wild gyrations. Sound familiar?

The economy won’t really bounce back until America’s surge toward inequality is reversed. Even if by some miracle President Obama gets support for a second big stimulus while Ben S. Bernanke’s Fed keeps interest rates near zero, neither will do the trick without a middle class capable of spending. Pump-priming works only when a well contains enough water.

[I]f Obama has declared war on inequality, inequality seems to be winning. In the deal he just cut with congressional Republicans, the president not only agreed to extend the Bush tax cuts for the highest earners but also to eliminate the estate tax for all but the microscopic percentage of people passing down more than $5 million—causing inheritance tax proponent Ray Madoff to declare the battle lost for good.