One hallmark of the first 30 years after World War II was the “countervailing power” of labor unions (not just at the bargaining table but in local, state, and national politics) and their ability to raise wages and working standards for members and non-members alike. There were stark limits to union power—which was concentrated in some sectors of the economy and in some regions of the country—but the basic logic of the postwar accord was clear: Into the early 1970s, both median compensation and labor productivity roughly doubled. Labor unions both sustained prosperity, and ensured that it was shared. The impact of all of this on wage or income inequality is a complex question (shaped by skill, occupation, education, and demographics) but the bottom line is clear: There is a demonstrable wage premium for union workers. In addition, this wage premium is more pronounced for lesser skilled workers, and even spills over and benefits non-union workers. The wage effect alone underestimates the union contribution to shared prosperity. Unions at midcentury also exerted considerable political clout, sustaining other political and economic choices (minimum wage, job-based health benefits, Social Security, high marginal tax rates, etc.) that dampened inequality. And unions not only raise the wage floor but can also lower the ceiling; union bargaining power has been shown to moderate the compensation of executives at unionized firms.

Over the second 30 years post-WWII—an era highlighted by an impasse over labor law reform in 1978, the Chrysler bailout in 1979 (which set the template for “too big to fail” corporate rescues built around deep concessions by workers), and the Reagan administration’s determination to “zap labor” into submission—labor’s bargaining power collapsed. The consequences are driven home by the two graphs below. Figure 1 simply juxtaposes the historical trajectory of union density and the income share claimed by the richest 10 percent of Americans. Early in the century, the share of the American workforce which belonged to a union was meager, barely 10 percent. At the same time, inequality was stark—the share of national income going to the richest 10 percent of Americans stood at nearly 40 percent. This gap widened in the 1920s. But in 1935, the New Deal granted workers basic collective bargaining rights; over the next decade, union membership grew dramatically, followed by an equally dramatic decline in income inequality. This yielded an era of broadly shared prosperity, running from the 1940s into the 1970s. After that, however, unions came under attack—in the workplace, in the courts, and in public policy. As a result, union membership has fallen and income inequality has worsened—reaching levels not seen since the 1920s.

By most estimates, declining unionization accounted for about a third of the increase in inequality in the 1980s and 1990s. This is underscored by Figure 2, which plots income inequality (Gini coefficient) against union coverage (the share of the workforce covered by union contracts) by state, for 1979, 1989, 1999, and 2009. The relationship between union coverage and inequality varies widely by state. In 1979, union stalwarts in the northeast and Rust Belt combined high rates of union coverage and relatively low rates of inequality, while just the opposite held true for the southern “right to work” states. A large swath of states—including the upper Midwest, the mountain west, and the less urban industrialized states of the northeast—showed lower-than-national rates of inequality at union coverage rates a bit above or a bit below that of the nation. More importantly, as we plot the same relationship in 1989, 1999, and 2009, those states move as a group towards the less-union coverage, higher-inequality corner of the graph. The relationship between declining union coverage and rising inequality is starkest in the earlier years (between 1979 and 1989). After 1999, union coverage has bottomed out in most states and changes in the Gini coefficient at the state level are clearly driven by other factors, such as financialization and the real estate bubble.

MORE: View interactive graphic of union decline and rising inequality in the U.S.

Colin Gordon is Professor of History at the University of Iowa and a Senior Research Consultant at the Iowa Policy Project