It is well established that unions provide benefits to workers — that they raise wages for their members (and even for nonmembers). They can help reduce inequality.

A new study suggests that unions may also help children move up the economic ladder.

Researchers at Harvard, Wellesley and the Center for American Progress, a liberal think tank, released a paper Wednesday showing that children born to low-income families typically ascend to higher incomes in metropolitan areas where union membership is higher.

The size of the effect is small, but there aren’t many other factors that are as strongly correlated with mobility. Raj Chetty of Stanford, Nathaniel Hendren of Harvard, and Patrick Kline and Emmanuel Saez of the University of California, Berkeley, who pioneered this method of examining economic mobility, established five factors that are strongly correlated with a low-income child’s likelihood of making it into the middle class: the rate of single motherhood in an area, the degree of inequality, the high school dropout rate, the degree of residential segregation, and the amount of social capital, as measured by indicators like voter turnout and participation in community organizations.

Single motherhood is the most strongly correlated factor with mobility. The latest study, which relied on the Chetty/Hendren data, says union membership is roughly as strongly correlated with mobility as the other four factors.