Most research examining growing income inequality in the United States has focused on economic causes, for seemingly obvious reasons.

But a new study suggests that a different cause – the politically induced decline in the strength of worker unions – may play a much more pivotal role than previously understood.

In fact, the role that union decline has played in growing income inequality may actually be larger than many of the favorite explanations offered by economists, such as the education gap in the United States.

Among their contributions to income equality: unions reduce pay differences within companies and use their influence to lobby on behalf of the working and middle classes, the researchers say. “The effect that unions used to have on protecting the incomes of middle class and working Americans has been underestimated,” said David Jacobs, co-author of the study and professor of sociology at The Ohio State University.

Jacobs conducted the study with Lindsey Myers, a doctoral student in sociology at Ohio State.Their results appear online in the journal American Sociological Review and are scheduled to appear in the August print edition.

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They concluded that unions likely would have lost members in the 1980s even if there had been presidents supportive of their cause, but the losses would have been less severe.

“After the Reagan turning point, unions no longer had the influence to help contain the acceleration in inequality,” Jacobs said.

How did unions help control inequality?

According to Jacobs, other research has shown that firms with unionized employees have diminished differences in pay – such that the gap in the earnings of the highest-paid worker and the lowest-paid workers was reduced in firms organized by unions.

“Unions were also the most effective political advocates forv the less affluent before Congress, the president and other elected officials,”Jacobs said. “They ended up helping less prosperous families even if they weren’t union members.”