When the Supreme Court badly weakened the Voting Rights Act in 2013, it described the landmark civil rights law as outdated. The formula that Congress had used back in 1965 to decide which areas of the country should have their voting laws placed under federal supervision no longer matched modern patterns of discrimination, Chief Justice John Roberts claimed.

“If Congress had started from scratch in 2006, it plainly could not have enacted the present coverage formula,” Roberts wrote for the majority, explaining why that formula was being struck down.

But a comprehensive new study by a renowned historian and expert in voting discrimination suggests what voting rights advocates have been saying all along: that Roberts got it wrong.

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Since 2009, Morgan Kousser, a professor of history and social science at the California Institute of Technology, has been collecting data on voting rights “events” across the country that resulted in a win for minorities. Kousser looked at over 4,100 voting rights lawsuits, inquiries by the Justice Department, and changes to laws in response to the threat of lawsuits, the great majority at the county level, stretching from 1957 to 2013.

Kousser found that over 90% of these events occurred in “covered jurisdictions”—that is, places that, until the Supreme Court’s decision, were required to submit their voting changes to the federal government for “pre-clearance.” The great majority of these jurisdictions were in the South.

The evidence, Kousser writes, “shows that the Chief Justice’s factual assertions were incorrect, that the coverage formula was still congruent with proven violations.”

Kousser also looked at cases brought under the VRA’s Section 2, which bars racial discrimination in voting, and was unaffected by the Shelby ruling. He found that from 1982—when Congress broadened Section 2—through 2005, 83.2% of successful Section 2 cases originated in covered jurisdictions, even though Section 2 covers the whole country.

Another key rationale that Roberts offered for striking down the formula was that the number of successful voting rights lawsuits has declined in recent years, rendering the Voting Rights Act’s pre-clearance system unnecessary. But Kousser offers compelling evidence that this has happened in large part because of a series of Supreme Court rulings, mostly in the 1990s, that narrowed the VRA’s scope—thereby creating a self-fulfilling prophecy about the decline of discrimination.

“[B]y rendering decisions that make it easier or harder to bring and win voting rights cases or make objections, the Supreme Court can, in effect, manipulate the evidence of discrimination, which it can then use, in a second stage, to justify a decision to further weaken or strengthen the tools,” Kousser writes. “It can create the reality that it subsequently reacts to.”

Kousser’s 1974 study of the Jim Crow-era disenfranchisement of southern blacks, “The Shaping of Southern Politics,” is considered a classic.

The new study doesn’t look at what happened after the Supreme Court’s decision. But if it had, it might have found more fodder for its case. Several southern states have moved forward with restrictive voting laws that had been on hold until the Shelby ruling ended federal oversight. One such law, Texas’s voter ID measure, was found by a federal judge last year to be intentionally discriminatory against minorities, but remains in place thanks to the Supreme Court. And numerous local jurisdictions have made changes to their election rules that, voting rights advocates say, have reduced minority political power.