Let it not be said that the editors of the Wall Street Journal lack ecological awareness. Today they save energy, both theirs and mine, by repeating exactly the same bogus arguments about income inequality that they made — and I refuted — fifteen years ago.

Let me just highlight what I had to say about essentially the same calculation highlighted by the chart in the middle of today’s piece:

We have finally come to the last, and perhaps most effectively confusing, conservative argument.

Let’s give the fact first: families who start out with high income on average have low or negative income growth over the next decade, while families who start out with low income on average see their incomes rise rapidly. This is true in both the Urban Institute and the Treasury data. In the Urban Institute’s numbers, families in the bottom quintile in 1977 saw their income rise 77 percent by 1986, while families in the top quintile saw their income rise only 5 percent. The editorial page of the Wall Street Journal, Paul Craig Roberts, and others have seized upon this kind of number as evidence that the poor actually did better than the rich in the 1980s. Let me call this the “WSJ calculation.”

The WSJ calculation seems striking; but on reflection it is completely consistent with the conclusion that the U.S. has rapidly growing inequality. It shows only that there is indeed some income mobility but nobody denied that. And it is no more a sign that supply-side policies helped the poor than the fact that very few people win the lottery several years in a row.

Unfortunately, it is hard to explain this without a numerical example: Imagine an economy in which in any given year half of the families earn $100,000 and the other half earn $200,000. And imagine also that this economy fits the blender model, so that a family that starts in the bottom half has a 50 percent chance of being in the top half ten years later, and conversely.

Now do the WSJ calculation. Families that start in the bottom half begin with $100,000; ten years later, on average they have $150,000, so they gain 50 percent. Families that start in the top half begin with $200,000; ten years later, on average they also have $150,000, so they lose 33 percent.

But has the distribution of income gotten more equal? No: it is unchanged. All that we see is the familiar statistical phenomenon of “regression toward the mean.” Essentially, the initially rich have nowhere to go but down, the initially poor nowhere to go but up. So if the income distribution were stable, any income mobility would inevitably produce the WSJ result; and it is not surprising that we still get it even when income inequality is rising.

Now, wasn’t that easy?

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