Until now, the study of unions’ effect on inequality has essentially started with the ’70s, because good data has been hard to come by for any time before then. But it was hard to tell a complete story about how the rise and fall of unions affected economic inequality because the data is confined to a time during which unions were already in decline.

In the new study, the four scholars have mined newly available Gallup Organization data going back to the 1930s, based on surveys of American households that include questions about political beliefs as well as union membership, education, and income. A rich trove of these older surveys is now publicly available at the Roper Center at Cornell University.

The four economists painstakingly cleaned and coded hundreds of these surveys spanning nearly 90 years. The data encompass the growth of unions during the 1930s and ’40s, their heyday in the ’50s and ’60s, and their slow decline to the present.

Union workers now earn about 20 percent more than nonunion workers in similar jobs. Remarkably, this union premium has held steady since the 1930s.

Throughout this period, the biggest boost from union membership has gone to the least educated workers, who have, in turn, driven the rise and fall of union membership. The decades following World War II, when unskilled workers formed the union movement’s backbone, marked the most rapid decreases in income inequality. Wages for nonwhite workers were particularly strong then.

But increasing wages for low-skilled union members is just one channel through which unions can reduce income inequality. Unions can also affect the earnings of nonunion workers.

To capture such effects, the researchers broadened their lens to include the entire distribution of workers and their wages beyond those who are in typically unionized jobs and industries. They found that, going back to the 1930s, more unions meant more income equality. During years and in states where workers were more likely to be unionized, income inequality was lower.