The high-profile fight to limit union power was replete with drama — including a recall election and state legislators fleeing to neighboring states.

In the 2011 battle in Wisconsin, Republican Gov. Scott Walker ultimately came out the victor. The controversial law passed, Walker won the recall, and the Democratic-aligned unions have lost much of their power.

But new research points to other losers in the fight: students in the state’s already struggling schools.

The first study to assess how Wisconsin’s high-profile weakening of unions, particularly teachers unions, affected students finds that it led to a substantial decline in test scores.

The findings come as the U.S. Supreme Court is set to hear arguments for a case, known as Janus, that could dramatically scale back union power across the country — essentially taking aspects of the Wisconsin model national. And they give credence to concerns from unions and their defenders that weakening teachers bargaining power would ultimately make schools worse, not better.

A report from the left-leaning Center for American Progress released Wednesday highlights this research — and the fact that teacher pay and average experience declined in the wake of the law, known as Act 10 — to argue that weakening unions ultimately harm schools.

“Those concerned about the quality of public education — and of all public services — should understand that Wisconsin’s Act 10 and associated budget cuts have not had the positive impact on education that its proponents claimed it would,” the CAP report argues.

Still, the research, which has not been formally peer-reviewed, only assesses the short-term impact of Wisconsin’s law. It adds to a complicated set of research findings on unions that doesn’t render a clear verdict.

Short-term effect in Wisconsin is negative, especially for low-achieving schools

The new research looks at the effects of Wisconsin Act 10, which became law in 2011 and severely limited the scope of collective bargaining and allowed members to opt out of unions.

The paper’s author, Jason Baron, took advantage of what was essentially a natural experiment set up by the law. Act 10 did not affect all school districts at once — a handful of school districts were allowed to maintain union rules until their existing contract expired up to two years later. That helped isolate the immediate impact of the law.

Baron found that weakening unions led to declines in test scores, particularly in math and science. The effects were fairly large, comparable to sharply increasing class sizes. And the harm was not evenly distributed: Schools that started out furthest behind were hurt the most, while higher achieving schools saw no impact.

Other research may help explain why.

The law led to big cuts in teacher compensation, particularly for veteran teachers and especially in health insurance and retirement benefits, according to one paper. There was also a spike in teacher retirement immediately following the law’s passage.

As compensation drops, it may become harder for district and teachers to recruit and keep teachers. An increase in retirement also reduces teacher experience, which has been linked to effectiveness.

Another study found that some Wisconsin districts moved from a single salary schedule to a performance-based pay system after Act 10’s passage. Those performance pay systems were more likely to be adopted by higher-achieving districts, potentially allowing them to lure effective teachers away from struggling schools.

“Following Act 10, high-performing schools filled vacancies from teacher retirements by poaching high-quality teachers from low-performing schools through attractive compensation schemes,” the paper concludes. So while those retirements might have hit all districts equally, high-performing districts were better able to make up the difference — at the expense of low-performing schools.

There is one study that complicates the narrative in Wisconsin. As retirements spiked, it found that academic achievement actually increased in the grades that teachers left. It’s not clear what explains this.

The larger question of how teachers unions affect learning remains up for debate

A number of other recent studies have examined the relationship between teachers unions and student outcomes outside of Wisconsin. The results aren’t consistent, but the trend has been more positive for unions of late. A caveat: Some of these studies have not been published in peer-reviewed academic journals.

  • On recent efforts to weaken unions: Research in Tennessee found that it led to a drop in teacher pay, but had no effect on student test scores. But a study of four states, including Wisconsin, that recently weakened unions found evidence of reduced teacher quality as a result.
  • On what happens when charter schools unionize: Two studies in California came to differing conclusions. One found that when charters unionize, student test scores go up, but the other showed no impact.
  • On the initial rise of collective bargaining: Another paper finds that students who went to schools where districts negotiated with unions earned less money and were more likely to be unemployed as adults. But this study looks at a fairly old data set — examining those who attended schools between 1965 and 1992.

Meanwhile, it’s not clear if any of this research is likely to influence the Supreme Court, as it considers the Janus case that could make life more difficult for unions. Last month, Chief Justice John Roberts called empirical studies on political gerrymandering “sociological gobbledygook.”