The Arts & Ideas section of today's New York Times carries an article about income inequality in America. The article begins, "For 30 years the gap between the richest Americans and everyone else has been growing so much that the level of inequality is higher than in any other industrialized nation."
The article goes on to quote from a book that claims, "Forty-seven percent of the total real income gain between 1983 and 1998 accrued to the top 1 percent of income recipients, 42 percent went to the next 19 percent, and 12 percent accrued to the bottom 80 percent."
Both statements assume a lack of income mobility. But that is at odds with reality. It doesn't make much sense to speak of the gap between "the richest Americans" and "everyone else" over a 30 year period without noting that a lot of people who 30 years ago were "the richest Americans" are now just "everyone else," and that a lot of people who 30 years ago were "everyone else" are now "the richest Americans." If you look at the people on the Forbes list of the richest Americans, many of those at the very top today -- Bill Gates, Paul Allen, Larry Ellison -- were in the "everyone else" category 30 years ago.
Similarly, the study that showed that "Forty-seven percent of the total real income gain between 1983 and 1998 accrued to the top 1 percent of income recipients" is almost certainly not based on a longitudinal study of what happened to the people who were the top 1 percent of income recipients in 1983, but on a snapshot comparison of the top 1 percent in 1983 against the top one percent in 1998. Those are different groups of people; whatever income gain is accruing is then being accrued in an important sense not by individuals but by categories. Which is an important distinction. The Times article talks about the effect of income inequality on "people's sense of satisfaction or dissatisfaction with their lot in life." You'd expect a poor person to be a lot less dissatisfied if he knew that in 20 years he could be a rich person -- or even if he knew that in 40 years his children or grandchildren could be rich -- than if he knew that he and his whole family would be mired in poverty for eternity.
Today's Times article does a decent job of talking to some conservative economists who make the argument that "The problem is not inequality but poverty." That's true as far as it goes. But discussing income inequality without discussing income mobility distorts the issue.