Americans cling to the conceit that they have unrivaled opportunity to move up and, alas, down the income ladder. This notion, that families that occupy the bottom rungs of the income ladder readily exchange places with those that occupy higher rungs, breeds indifference to the most glaring economic fact of the past quarter-century: widening inequality. The gap in earnings, for example, between college and high school graduates more than doubled.

Yet in a recent Op-Ed piece in The New York Times, W. Michael Cox and Richard Alm acknowledge that the wealth of poor families has been falling at the same time that the wealth of rich families has been rising, and ask, "So what?"

Mr. Cox and Mr. Alm suggest that mobility along the income ladder is serving as an antidote to inequality. But exactly how much mobility is there? According to them, quite a lot. "America isn't a caste society," they say, citing a study by the Federal Reserve that shows that only 5 percent of the poorest 20 percent of people stayed in that lowest income group over a 17-year period.

Everyone else moved higher.

But this stunning statistic is stunningly misleading.

Prof. Peter Gottschalk of Boston College points out that the Fed study counts teenagers and young adults from upper-income families who work part time while in school as moving upward when they find higher-paying jobs after graduation. This upward mobility of students hardly answers the enduring question: How many grown-ups are trapped in low-paying jobs?

The answer is, a lot. America's conceit is just that -- a conceit.

Mr. Gottschalk and Prof. Sheldon Danziger of the University of Michigan have looked at the plight of children during the 1970's and 80's. They separate children into five groups according to their family's income. About 6 in 10 of the children in the lowest group -- the poorest 20 percent -- in the early 1970's were still in the bottom income group 10 years later.

Almost 9 in 10 children in the bottom group remained in the bottom two income groups 10 years later.

Neither of these figures budged in the 1980's, providing solid evidence that mobility did not rise over the 20-year period. About 70 percent of poor black children in the early 1970's were still poor a decade later.

The upshot is that few families occupying the low rungs of America's income ladder get rich.

Economists provide no rule for figuring out how much mobility a society needs to counter the impact of any given amount of inequality. But there is indirect evidence that mobility is not high enough in this country to provide an adequate safety valve for those on the bottom rungs. Even if mobility was high enough 30 years ago to justify a so-what attitude toward inequality, it seems improbable that it could be high enough today, given that mobility has remained constant but inequality has reached extreme levels. Another sobering indicator: mobility is no higher in the United States than in Western Europe, where living standards are comparable and there is only a fraction as much inequality.

Some inequality is essential for capitalism. High incomes are the reward for studying long hours, working hard and investing in risky businesses. But high incomes can result for reasons that have little to do with merit, reflecting rich parents, luck or manipulation of the political process. Inequality driven by such factors can breed social division, anger and a politics of resentment. It also belies the notion that America gives everyone a fair chance.

Economists are in general reluctant to tamper with market-driven incomes. The risk of squashing incentives and innovation can be great. But there is a compelling case for eradicating one virulent form of inequality: poverty. The richest country in the history of the universe tolerates a poverty rate of about 20 percent among its children, and about 35 percent among its black children. No conceit about mobility, real or imagined, can excuse that unconscionable fact.

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Copyright 2000 The New York Times Company