There’s something curious about the labor force in the United States. Identical jobs and industries have become unionized in some states while remaining nonunionized in others. Unionization levels vary greatly from state to state.

As of 2013, 20 to 25 percent of employed residents in some states belonged to a union, while in other states the union density was only 3 to 5 percent.

According to the Bureau of Labor Statistics, in 2015 roughly half of the nation’s 14.8 million union members lived in just seven states: California, New York, Illinois, Pennsylvania, Michigan, Ohio, and New Jersey. Yet these states accounted for only one-third of paid employment nationally. And those same states have held the highest unionization rates for decades. So why do some states remain heavily unionized while others do not? “It turns out there was something that happened in the 1930s that set the rank of unionization in place across states in the United States, and that rank has stayed roughly the same ever since,” says Lauren H. Cohen, the L.E. Simmons Professor of Business Administration at Harvard Business School.

The thing that happened was the Dust Bowl: a series of severe dust storms and droughts that decimated farms in the Great Plains during the 1930s, forcing thousands of families to abandon their property. Many migrated to close-by cities, often in California but also in other states, in hopes of finding jobs.

Alas, the Dust Bowl coincided with the Great Depression. Jobs were scarce, and those who still had jobs were loath to lose them to migrants. And so they unionized.

“Let’s say you were a subsistence farmer,” Cohen explains. “The drought dried up your crops. You still had to feed your family. So you traveled to the closest city and tried to get a job. Of course, that put pressure on people who did have jobs. They were working for a dollar an hour, and you were willing to come and do the same job for 50 cents. So the people who had jobs said, ‘Let’s unionize to make sure these farmers don’t take our jobs.’”