Poverty and income inequality over time

Justin Wolfers posted this graph at Freakonomics, showing income growth by quintile since the 1970s:

His analysis was that economic growth has done little for the poor, accruing mostly (almost entirely) to the upper incomes.

Russ Roberts, however, takes issue with this analysis. In two separate posts (here and here), he criticizes Wolfers' interpretation:

"This alas, is a meaningless chart. It tells you nothing about who got the gains of the last 35 years. Why? Because they're not the same people in the quintiles. Starting in 1973, and it's not a coincidence, the divorce rate in the United States began to rise. The number of families increased dramatically simply because of divorce. There was also an increase in the number of families headed by single women with children. The quintile breaks-points changed, not because the economy was growing or shrinking but simply because of changes in the types of families."

In a sense, Roberts is not wrong. Looking at static graphs of quintiles does not account for differences in composition. If people move up the income ladder, then we will mistake static income growth among the groups for static income growth among individuals. The key here is the degree of social mobility. Basically, what are the odds that a person who is poor today will be poor next year, or in ten years?

Emmanuel Saez, the most recent winner of the John Bates Clark Medal, looks at this question in depth. Using Social Security data, Seaz tracked individuals over time to assess income growth and social mobility in the US over time. He concludes:

"We found that changes in short-term mobility have not substantially affected the evolution of inequality, so that annual snapshots of the distribution provide a good approximation of the evolution of the longer term measures of inequality. In particular, we find that increases in annual earnings inequality are driven almost entirely by increases in permanent earnings inequality with much more modest changes in the variability of transitory earnings. However, our key finding is that while the overall measures of mobility are fairly stable, they hide heterogeneity by gender groups. Inequality and mobility among male workers has worsened along almost any dimension since the 1950s: our series display sharp increases in annual earnings inequality, slight reductions in short-term mobility, large increases in long-term inequality with slight reduction or stability of long-term mobility.

Against those developments stand the very large earning gains achieved by women since the 1950s, due to increases in labor force attachment as well as increases in earnings conditional on working. Those gains have been so great that they have substantially reduced long-term inequality in recent decades among all workers, and actually almost exactly compensate for the increase in inequality for males."

Certainly, some longitudinal studies have found larger amounts of social mobility, but the bulk of the literature has not found the degree of mobility that Roberts implies.

So while the graph posted by Wolfers is static, there is evidence that it is a good approximation of reality. Income growth has accrued largely to wealthier individuals and families over the past 30 years, and it has not been sufficiently counterbalanced by social mobility.

Curiously, Roberts says that liberals lack a causal mechanism for changes in inequality and stagnant incomes among the poor. However, Wolfers was explicitly citing one of the most powerful arguments for this phenomenon, namely "skill-biased technological change". As Harvard's Larry Katz and Claudia Goldin explain, technological change has raised the productivity of skilled workers relative to less skilled workers, causing changes in relative incomes. In manufacturing, for example, advanced machinery has decreased demand for assembly line workers, but increased demand for engineers and mechnical operators. At the same time, the supply of educated workers has not kept up, resulting in large income gains in the upper half of the income distribution and much lower gains at the bottom.

Roberts is correct, however, in asserting that there is a difference between income inequality and absolute well-being. A rising tide can lift all boats, even while it lifts some faster than others. But it's important not to minimize slow income growth among lower income Americans; they may be better off than they were 30 years ago, but they are still struggling.

In a follow-up post, I'll delve more into the measurement, economics and politics of inequality.