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.

Personally, I think that these disparities of wealth should not be reproduced in the political arena, and that in a truly (hence impossibly) fair democracy, David Koch would literally find it no easier to get the governor of Wisconsin on the phone than my cousin Lisa would. (Actually, it should be easier for Lisa, since she’s a Wisconsin voter.) To get anywhere near that kind of fairness, we would have to institute radically different rules for the political game in America, and I don’t expect my colleague to share all my beliefs about how to weigh the political equality of citizens against the freedom of billionaires to spend their money on politics. But I do think that if we don’t agree that rich people have more political power than poor people and that they use that power to pursue their economic interests, then we’ve really got a communications problem.

IF YOU HAPPENED to be watching NBC on the first Sunday morning in August last summer, you would have seen something curious. There, on the set of Meet the Press, the host, David Gregory, was interviewing a guest who made a forceful case that the U.S. economy had become “very distorted.” In the wake of the recession, this guest explained, high-income individuals, large banks, and major corporations had experienced a “significant recovery”; the rest of the economy, by contrast—including small businesses and “a very significant amount of the labor force”—was stuck and still struggling. What we were seeing, he argued, was not a single economy at all, but rather “fundamentally two separate types of economy,” increasingly distinct and divergent.

This diagnosis, though alarming, was hardly unique: drawing attention to the divide between the wealthy and everyone else has long been standard fare on the left. (The idea of “two Americas” was a central theme of John Edwards’s 2004 and 2008 presidential runs.) What made the argument striking in this instance was that it was being offered by none other than the former five-term Federal Reserve Chairman Alan Greenspan: iconic libertarian, preeminent defender of the free market, and (at least until recently) the nation’s foremost devotee of Ayn Rand. When the high priest of capitalism himself is declaring the growth in economic inequality a national crisis, something has gone very, very wrong.