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Mobility and Income Distribution

15 一月, 2016 - 09:47

When we speak of the bottom 20% or the middle 20% of families, we are not speaking of a static group. Some families who are in the bottom quintile one year move up to higher quintiles in subsequent years; some families move down. Because people move up and down the distribution, we get a quite different picture of income change when we look at the incomes of a fixed set of persons over time rather than comparing average incomes for a particular quintile at a particular point in time, as was done in Figure 19.1.

Addressing the question of mobility requires that researchers follow a specific group of families over a long period of time. Since 1968, the Panel Survey of Income Dynamics (PSID) at the University of Michigan has followed more than 5,000 families and their descendents. The effort has produced a much deeper understanding of changes in income inequality than it is possible to obtain from census data, which simply take a snapshot of incomes at a particular time.

Based on the University of Michigan’s data, Federal Reserve Bank of Boston economists Katharine Bradbury and Jane Katz compared mobility in the 1970s, 1980s, and 1990s. In the 1970s, just under half the families in the poorest quintile at the start of that decade were still in that quintile at the end of that decade and overall about 32% of families moved up one quintile or more. The mobility figures for the 1980s were about the same as for the 1970s. In the 1990s, however, mobility declined. About 30% of families moved up one quintile or more and 53% of families that started the 1990s in the poorest quintile were still in that quintile at the end of the 1990s. In every decade, some of the movement to higher quintiles results simply from gaining age and experience. The researchers further comment that, for the 1990s, moving across quintiles has become harder to achieve precisely because of the increased income inequality.Katharine Bradbury and Jane Katz, “Issues in Economics: Are Lifetime Incomes Growing More Unequal? Looking at New Evidence on Family Income Mobility,” Regional Review 12:4 (4thQuarter, 2002): 2–5.