Joining a union is the best investment a worker can make.

Unions need defending, maybe more than ever, because of the attacks they face. The passage of a right-to-work law in Wisconsin and Illinois Gov. Bruce Rauner's proposal for union-free zones show how distorted the lens is when the focus turns to organized labor.

Right-to-work laws are intended to limit union growth, but advocates never cite political motives or antipathy for working people. Instead, their calls for reducing labor market protections are based on the claim that unions restrain personal liberty and restrict economic development.

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Nothing is further from the truth.

The "labor hater," as Martin Luther King Jr. once called the corporate and political conservatives who mobilize against organized labor, argues that if you reduce unionization, economic prosperity will be unleashed. Yes, but for whom? Restricting union growth has always been bad for workers' economic and political freedom. The cumulative weight of decades of social science has unquestionably demonstrated that union-bargained contracts provide workers with higher incomes, more and better benefits, and a stronger "voice" in the workplace.

Implementing a statewide right-to-work law in Illinois would be punitive for working men and women. According to a 2013 University of Illinois study that I co-authored, workers would suffer an income loss of 5.7 percent to 7.3 percent. Additionally, fewer workers would have health and retirement benefits, and with workers earning less, poverty would likely rise by 1 percent.

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As King warned in the 1960s, after mostly Southern states moved to adopt right-to-work, the losses would be particularly harsh on people of color. Per-hour work incomes are at least $2.49 lower in right-to-work states for African-American, Latino and Asian workers, compared with their wages in collective bargaining states. With lower earnings, annual state income tax revenues in Illinois would shrink by $1.5 billion.

To be fair, Rauner has not called for a statewide law. So what would the effects of a more limited local jurisdiction approach be on Illinois workers? The premise of the local zones is that unionization suppresses job growth. But like so many claims for opposing policies that protect workers, the criticism doesn't hold up.

A look at recent data for the Chicago area suggests that union membership levels have no direct correlation to higher unemployment. The opposite's true, in fact. Around Chicago in 2013, the county with the fewest union members had the six-county area's highest unemployment rate.

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When you look more broadly, you find that the average unemployment rate for all eastern Illinois counties bordering right-to-work Indiana was 5.7 percent, compared with 7.6 percent for those Indiana counties just across the border. And while right-to-work prophets predict a paradise of unparalleled job creation, in 2014, Illinois added 103,000 jobs (fourth highest in the nation), compared with Indiana's 89,000.

Union defenders should never suggest that collective bargaining is either the primary or sole driver of job creation; nor should right-to-work supporters argue that limiting union dues is a sure-fire way to put people to work.

What is assured is that the loss of income that would result from a reduction of union members will exacerbate existing income disparities. If just half of Illinois' counties transitioned into "union free zones," total employee compensation would drop an estimated $1.2 billion.

It's also possible that with or without right-to-work, employment could spike in Illinois. For example, the state could take up large-scale hydraulic fracturing. But no matter the reasons that jobs appear, what is important is how the workers are valued.

Robert Bruno is a professor of labor and employment relations at the University of Illinois at Urbana-Champaign. He wrote this for the Chicago Tribune.