Penn economist links ‘assortive mating’ and income inequality

Assortive Mating

Income inequality in the United States is widening in part because of “assortive mating,” says a Penn economist—the tendency for high-income, college-educated men and women to marry each other instead of marrying low-income individuals without college degrees.

Jeremy Greenwood, a professor in the Department of Economics in the School of Arts & Sciences, co-authored the new National Bureau of Economic Research working paper, “Marry Your Like: Assortive Mating and Income Inequality.” He says assortive mating is an economics term for the non-random selection of a spouse with the same level of education.

Greenwood’s study, completed with colleagues from Markets, Organizations and Votes in Economics in Spain, and the University of Konstanz and University of Mannheim in Germany, answers two main questions: (1) Has there been an increase in positive assortive mating? (2) Does assortive mating contribute to household income inequality? 

The researchers used United States Census Bureau data from 1960-2005 to provide a snapshot of hundreds of thousands of households.

In 1960, a household at the top 10 percent earned about three-and-a-half times the average U.S. income. In 2005, the top 10 percent-earning households made more than four times the average U.S. household income.

“Incomes are more polarized in 2005,” write Greenwood and his co-authors. “The change in wages across individuals is the primary driver of this increase in income inequality.” 

The Gini Index, a tool that measures a society’s inequality on a scale of 0 to 1, was central to the study. A measure of 0 means all income is equally distributed; a measure of 1 means only one person has all of the income.

“In 1960, the Gini coefficient was 0.34,” says Greenwood, who is also a research associate at Penn’s Population Studies Center. “In 2005, the Gini coefficient was 0.43. If spouses had been chosen at random, the Gini coefficient would have been the same as it had been in 1960.”

In 1960, about 38 percent of women worked outside the home, but by 2005, that figure had climbed to 59 percent. More women were in the labor force and more women held college degrees, which led to higher salaries.

Jeremy Greenwood

Penn Economics

Jeremy Greenwood, a professor in the Department of Economics in the School of Arts & Sciences.

The researchers found that in 2005, more people were interested in seeking mates with similar education and income. With more women in the workforce with college degrees, there were more two-income households of professionals with college degrees. These highly educated husbands and wives earned several times more than married couples without college degrees, increasing income inequality.

Greenwood says the upshot of the study is that assortive mating will be an important factor in determining the distribution of household incomes in the U.S. economy.  

“There is a marked tendency for individuals to marry others with the same educational background,” he says. “Perhaps this is because they want a mate from the same social class, or because they simply meet similar types either at school or on the job, or because they desire a certain standard of living. Whatever the reason, such assortative mating operates to increase income inequality across American households.”

Originally published on February 13, 2014