Changes in family structure may explain anywhere from 15 to 40 percent of the increased inequality in recent decades. Readers may wonder why there is such a broad range of estimates. It depends on the time period examined, the income rungs examined, and assumptions about how much the absent parent might have brought into the household.
Mr. Western’s estimate that the rise in single parenthood explains 21 percent of the growth in inequality comes from a 2008 article in the American Sociological Review (with Christine Percheski and Deirdre Bloom). He examined the change from 1975 to 2005.
Gary Burtless looked at different years (1979 to 1996) for the European Economic Review but came up with the same figure: 21 percent. He also found that the increased tendency of educated people to marry each other accounted for another 13 percent of inequality’s growth.
The other estimate cited in the article comes from Robert Lerman of the Urban Institute. His unpublished analysis examines families with children at the 25th percentile and the 75th percentile. In 1975, the higher group had 2.16 times the income of the lower group. By 2008, it had risen to 3.09. Mr. Lerman estimated that 40 percent of that rise was the result of increasing single parenthood.