income mobility

The Treasury recently came out with a study of income mobility in the US that is receiving a lot of play in the Wall Street Journal and in several blogs.

Income mobility studies are very difficult to do properly and often are very misleading because they incompletely adjust for different factors.

The biggest factor driving income mobility in the US is age. People start working as young people and as they gain experience and earn promotions they receive higher incomes. The Census Bureau reports data that shows how this process. According to their data in 2006 median incomes by age bracket were:




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0VER 65



2006 is typical or normal. If you looked at the data in 1975, 1985 or 1995 or any year in-between you would find a very similar pattern of income growing sharply as people age and starting to fall after the age of 55 and falling very sharply after age 65 when most individuals retire.

The Treasury study looked at income tax returns of people over 25. In this way they avoided the problem of showing the Doctors daughter making $5,000 at age 20 with her summer job as a life guard at the country club and $50,000 at age 30 as a drug company representative.

Income mobility is commonly described as an escalator moving everyone on the escalator up over time. It is a good analogy, and the three factors drive this move. One is economic growth. If you take a snapshot of everyone on the escalator in 1995 and another snapshot in 2005 you can see how the average has changed and get a picture of how economic growth influence the location of the escalator.

The second factor is the aging of the population. This is what dominates this Treasury study. They took a sample of the population in 1995 and came back and looked at the same individuals in 2005 when everyone in the sample was ten years older. This approach deals with some of the problems in the first methodology where the comparison between 1995 and 2005 would look at different people. In the first methodology in 2005 the people in the 15-24 year old age bracket were not included in the 1995 snapshot and the 15-24 year olds are now in the 25-34 year bracket.

There is nothing wrong with this methodology and it deals with some of the problems in the first approach. It is a valid approach. But you have to be careful in evaluating what the results mean. For example, if you look at what happens to the income of the lowest quintile you see that their income grows very rapidly. Their incomes will grow rapidly because of three factors. One is overall economic growth that moves everyone higher. A second is age which will move everyone in the 25-34 age bracket into the 35-44 age bracket, etc (See below).. A third is what we normally think of as economic mobility as individuals through hard work, education and good fortune or luck improve themselves. But since this study is dominated by people aging the lowest income group of the 25-34 age bracket in 1995 and in the 35-44 age bracket in 2005. Given that it is normal for the average income of the 35-44 age bracket to be around 25% higher than the 25-34 age bracket the study finding that over half the individuals in the lowest quintile moved to another quintile is not surprising. What is surprising to me is that about half of the lowest quintile were still in the lowest quintile ten years later.
















over 65


The problem with the Wall Street Journal and several blogs analysis of this data is that they tend to credit all the change from 1995 to 2005 to overall economic growth and/ or
individual economic mobility. But this is misleading as the dominant factor driving the changes reported in the Treasury study is the aging of the population. But we do not have the data to age adjust the data to see how much of the reported improvement was due to this factor.

This is not to say that there is anything wrong with the Treasury study, It is a common shortcoming of many such studies. That is the reason why the best studies on income mobility are the new series of studies that look at lifetime earnings and compare lifetime earnings of one generation to the lifetime earnings of their parents generation.

But it does mean that many of the advances in average income reported and incomes of the lowest income quintiles emphasized in the Wall Street Journal discussion of the study are misleading. They are attributing the sharp growth in the income of the lowest income brackets to overall economic growth and income mobility when it is largely due to people moving into the next age bracket. But they conveniently fail to point this out.