On this week’s Political Scene podcast, Dorothy Wickenden asked me why inequality has suddenly emerged as a live political issue. After ignoring the subject for years, even Republicans are being forced to address it. Eric Cantor, the House Majority Leader, is set to deliver a speech today about income disparity and ideas for promoting social mobility.

On one level, the answer to Dorothy’s question is obvious: the reason inequality is a big deal is because it has risen so much. A new chart from Lawrence Mishel, of the liberal-leaning Economic Policy Institute, shows that between 1979 and 2007 people in the top one per cent of the economic pile saw their incomes double, whereas most everybody else hardly gained at all. (For those at the very, very top of the income distribution—the top 0.1 per cent—things were even better: their incomes quadrupled.)

A second way to tell this story is to look at who gets to keep the extra dollars that the economy generates over the years as it expands. Mishel has produced another useful chart. It shows that between 1979 and 2007 almost two-thirds of all income growth (63.7 per cent) went to those in the top ten per cent of the income distribution. Very little of it—just eleven per cent—went to those in the bottom three-fifths of the income distribution (i.e. the majority of folks).

Looking at charts like these, the pertinent question isn’t why Americans are increasingly concerned about inequality, but why they weren’t marching in the streets years ago. The stagnation of wages and incomes for working-class and lower-middle-class Americans goes back to the nineteen-seventies, and the enormous gains at the top began in the nineteen-eighties.

It isn’t as if nobody noticed what was happening. My first article about the subject appeared in 1995, and I was late to the game. Recently, I was re-reading a September 1992 piece in the American Prospect by Paul Krugman, which provided an excellent survey of developments up to that date. Rising inequality actually featured fairly prominently in the 1992 Presidential campaign. But with Bill Clinton’s election, and particularly with the economic boom of the late nineteen-nineties, which lifted all boats, it gradually receded as a political issue. Now it’s back with a vengeance, and not just because of Occupy Wall Street. Even among middle-aged suburbanites who wouldn’t dream of going down to Zuccotti Park the lopsided allocation of rewards in today’s economy seems to be causing concern: hence the reaction of Cantor and other Republicans.

Ironically, this is happening when, strictly speaking, income inequality may well be falling, at least according to some commonly used measures. On her blog at TheAtlantic.com a couple of days go, Meghan McArdle presented two interesting charts based on some recent empirical work by Steven Kaplan, an economist at the University of Chicago. The charts show how the share of total income taken by the top one per cent of the income distribution and the top 0.1 per cent has changed over the decades. From 1973 to 2007, the lines rise steadily, except during recessions, when they fall back. In 2008 and 2009, the most recent year for which Kaplan could find figures, the lines fall back again, and quite steeply.