In a “letter from the editor” last week, The New York Times' David Leonhardt claimed that liberals overlook evidence that changing household structure, meaning the relative decline of households headed by a married couple, increases inequality. He wrote, “I’d say that family structure is an area where many liberals are putting more weight on their preconceptions (inequality is bad for society) than on the evidence (changes in family structure are both an effect and a cause of inequality).” But there is substantial evidence that household structure is what’s known as an endogenous variable: People make marriage, divorce, and child-rearing decisions due to their economic circumstances. Furthermore, the causes of rising inequality are the same reasons why household structure has changed: stagnant wages, labor market detachment, and job lock.

When questioned on Twitter about the evidence for his claim that changes in family structure cause inequality to rise, Leonhardt cited a paper by Professor Molly Martin of Penn State. But Martin argues nothing of the sort.

Her paper, "Family structure and income inequality in families with children, 1976 to 2000," essentially asks this question: "To what extent is rising inequality due to growing inequality between types of households (e.g., the gap between married couples and single mothers has grown, and a larger share of households belong to the more disadvantaged group), and to what extent is it attributable to growth of inequality within types of households?” Martin finds that 41 percent of the increase in household inequality from 1976-2000 is attributable to changing family structure.

But in a section entitled “Limitations,” Martin writes, “First, the results do not document causal relationships. I cannot determine the degree to which family structure changes caused the observed changes in inequality.” And Martin concludes an appendix further addressing these issues with “family formation probably reacts to prevailing economic conditions and, in that response, sets the conditions for perpetuating broader inequality patterns.” A single clause from the abstract of her paper, "family structure shifts explain 41% of the increase in inequality,” has been incorrectly cited as evidence that the decline in married-parent households caused inequality to increase, when the author herself disavowed that interpretation both in the paper and thereafter. She notes that, “the relationship between family formation behavior and inequality appears to be declining over time” and even during the period where it was most influential, it accounted for very little of the change. See her chart:

There are further problems with using Martin’s paper as the basis for the argument that changing family structure caused inequality to rise. She uses survey data that top-code the highest income households. That means that households earning more than some threshold are reported as earning at the threshold in order to guard their confidentiality. The Current Population Survey is considered to be informative about the bottom 98 percent of the income distribution (more or less), but the top is where all the action has happened, so to speak. Thomas Piketty and his coauthors have shown the rise of top income shares in numerous papers, and it is why they have developed the World Top Incomes Database to track the incomes of the richest.