For about a century, economic inequality has been measured on a scale, from zero to one, known as the Gini index and named after an Italian statistician, Corrado Gini, who devised it in 1912, when he was twenty-eight and the chair of statistics at the University of Cagliari. If all the income in the world were earned by one person and everyone else earned nothing, the world would have a Gini index of one. If everyone in the world earned exactly the same income, the world would have a Gini index of zero. The United States Census Bureau has been using Gini's measurement to calculate income inequality in America since 1947. Between 1947 and 1968, the U.S. Gini index dropped to .386, the lowest ever recorded. Then it began to climb.
Income inequality is greater in the United States than in any other democracy in the developed world. Between 1975 and 1985, when the Gini index for U.S. households rose from .397 to .419, as calculated by the U.S. Census Bureau, the Gini indices of the United Kingdom, the Netherlands, France, Germany, Sweden, and Finland ranged roughly between .200 and .300, according to national data analyzed by Andrea Brandolini and Timothy Smeeding. But historical cross-country comparisons are difficult to make; the data are patchy, and different countries measure differently. The Luxembourg Income Study, begun in 1983, harmonizes data collected from more than forty countries on six continents. According to the L.I.S.'s adjusted data, the United States has regularly had the highest Gini index of any affluent democracy. In 2013, the U.S. Census Bureau reported a Gini index of .476.
The evidence that income inequality in the United States has been growing for decades and is greater than in any other developed democracy is not much disputed. It is widely known and widely studied. Economic inequality has been an academic specialty at least since Gini first put chalk to chalkboard. In the nineteen-fifties, Simon Kuznets, who went on to win a Nobel Prize, used tax data to study the shares of income among groups, an approach that was further developed by the British economist Anthony Atkinson, beginning with his 1969 paper "On the Measurement of Inequality," in the Journal of Economic Theory. Last year's unexpected popular success of the English translation of Thomas Piketty's "Capital in the Twenty-first Century" drew the public's attention to measurements of inequality, but Piketty's work had long since reached American social scientists, especially through a 2003 paper that he published with the Berkeley economist Emmanuel Saez, in The Quarterly Journal of Economics. Believing that the Gini index underestimates inequality, Piketty and Saez favor Kuznets's approach. (Atkinson, Piketty, Saez, and Facundo Alvaredo are also the creators of the World Top Incomes Database, which collects income-share data from more than twenty countries.) In "Income Inequality in the United States, 1913-1998," Piketty and Saez used tax data to calculate what percentage of income goes to the top one per cent and to the top ten per cent. In 1928, the top one per cent earned twenty-four per cent of all income; in 1944, they earned eleven per cent, a rate that began to rise in the nineteen-eighties. By 2012, according to Saez's updated data, the top one per cent were earning twenty-three per cent of the nation's income, almost the same ratio as in 1928, although it has since dropped slightly.
Political scientists are nearly as likely to study economic inequality as economists are, though they're less interested in how much inequality a market can bear than in how much a democracy can bear, and here the general thinking is that the United States is nearing its breaking point. In 2001, the American Political Science Association formed a Task Force on Inequality and American Democracy; a few years later, it concluded that growing economic inequality was threatening fundamental American political institutions. In 2009, Oxford University Press published both a seven-hundred-page "Handbook of Economic Inequality" and a collection of essays about the political consequences of economic inequality whose argument is its title: "The Unsustainable American State." There's a global version of this argument, too. "Inequality Matters," a 2013 report by the United Nations, took the view-advanced by the economist Joseph Stiglitz in his book "The Price of Inequality"-that growing income inequality is responsible for all manner of political instability, as well as for the slowing of economic growth worldwide. Last year, when the Pew Research Center conducted a survey about which of five dangers people in forty-four countries consider to be the "greatest threat to the world," many of the countries polled put religious and ethnic hatred at the top of their lists, but Americans and many Europeans chose inequality.
What's new about the chasm between the rich and the poor in the United States, then, isn't that it's growing or that scholars are studying it or that people are worried about it. What's new is that American politicians of all spots and stripes are talking about it, if feebly: inequality this, inequality that. In January, at a forum sponsored by Freedom Partners (a free-market advocacy group with ties to the Koch brothers), the G.O.P. Presidential swains Ted Cruz, Rand Paul, and Marco Rubio battled over which of them disliked inequality more, agreeing only that its existence wasn't their fault. "The top one per cent earn a higher share of our income, nationally, than any year since 1928," Cruz said, drawing on the work of Saez and Piketty. Cruz went on, "I chuckle every time I hear Barack Obama or Hillary Clinton talk about income inequality, because it's increased dramatically under their policies." No doubt there has been a lot of talk. "Let's close the loopholes that lead to inequality by allowing the top one per cent to avoid paying taxes on their accumulated wealth," Obama said during his State of the Union address. Speaker of the House John Boehner countered that "the President's policies have made income inequality worse."
The reason Democrats and Republicans are fighting over who's to blame for growing economic inequality is that, aside from a certain amount of squabbling, it's no longer possible to deny that it exists-a development that's not to be sneezed at, given the state of the debate on climate change. That's not to say the agreement runs deep; in fact, it couldn't be shallower. The causes of income inequality are much disputed; so are its costs. And knowing the numbers doesn't appear to be changing anyone's mind about what, if anything, should be done about it.
The growth of inequality isn't inevitable. But, insofar as Americans have been unable to adopt measures to reduce it, the numbers might seem to suggest that the problem doesn't lie with how Americans treat one another's kids, as lousy as that is. It lies with Congress.