This post is part of Polyarchy , an independent blog produced by the political reform program at New America , a Washington think tank devoted to developing new ideas and new voices.

In 2002, North Carolina began a public policy experiment that asked: What would happen if you had a judicial system where judges did not have to raise campaign money from lawyers and clients who had business before the court?

The answer, it turns out, is pretty much what common sense would dictate: You'd get justices who were more impartial, and less likely to vote in favor of attorneys who donated to their campaigns.

That's the conclusion from just-published paper "Does Public Financing Affect Judicial Behavior? Evidence From the North Carolina Supreme Court," by political scientists Morgan L.W. Hazelton, Jacob M. Montgomery, and Brendan Nyhan.

In addition to finding evidence that justices who opted in to public financing became "relatively less favorable toward attorney donors," the paper also shows that justices who opted in to the public funding system became more moderate in their voting patterns.

Unfortunately, North Carolina did away with this public funding system in 2013, as part of a sweeping reform package that also made it harder to vote. But at least we had 11 years, enough to know that the system worked.

What happened in North Carolina?

The North Carolina system was voluntary, so judicial candidates could choose to forgo most private donations in exchange for public support. The system was paid for by a $50 assessment on all attorneys and a voluntary $3 check-off on the state income tax forms. Between 2004 and 2010, 47 of 61 (77 percent) of candidates facing contested elections in either the state Supreme Court or the Court of Appeals chose the public funding option. Both Republicans and Democrats used the system.

Hazelton, Montgomery, and Nyhan examined the behavior of a smaller subset of justices on the state Supreme Court whose service spanned the implementation of the law. They looked at how often the justices voted in favor of the attorneys who were arguing cases before them, and how that behavior changed after the justices stopped relying on those attorneys for donations.

As compared with the justices who continued to rely exclusively on private donors for money, the justices in the public funding system were 60 percent less likely to vote in favor of donors who contributed money to their campaigns at some point in the previous eight years. (The researchers limited their analysis to the cases that were non-unanimous, on the reasonable assumption that the split decisions were harder calls where campaign contributions might be more important).

This 60 percent change is a pretty significant effect. Opting in to the public fundraising system clearly made judges much more impartial — exactly the quality we most want in our judges.

The nice thing about this study is that it gets past the "endogeneity problem" that is often present in analyses trying to show the effect of campaign contributions on outcomes. It's easy to correlate contributions with votes, both in Congress and in the courts. But it's much harder to show that contributions caused votes, since it's also possible that contributors just give to their natural allies and friends. Causation assumes that absent the contribution, lawmakers or judges would have voted differently. Of course, there are many ways in which money may indirectly influence the positions one takes, or who gets into office in the first place, without directly altering votes. But these are much more difficult to prove statistically, in part because it's hard to find relevant counterfactual observations to compare against.

The advantage of Hazelton, Montgomery, and Nyhan's approach, at least from a research perspective, is that it effectively compares the same judges before and after they opted in to public funding, and it shows that public funding freed these justices from what appears to have been observable favoritism towards those who had contributed to their campaigns.

Should justices be allowed to fundraise privately?

The obvious question this raises is why we allow judicial elections to be privately funded at all.

In a recent Supreme Court decision (Williams-Yulee v. Florida Bar), a 5-4 majority upheld a Florida law that prohibited elected judges from actively fundraising. In writing the majority opinion, Justice John Roberts argued, "Judges are not politicians, even when they come to the bench by way of the ballot. ... A state may assure its people that judges will apply the law without fear or favor — and without having personally asked anyone for money." (Of course, the law doesn't ban justices from collecting money — it just prevents them for asking for it directly. But still, it's a step in the right direction)

The logic here is pretty simple. As state judicial elections get more expensive (which they have), justices become more reliant on campaign cash. The major suppliers of that campaign cash tend to have business before the courts. As Hazelton, Montgomery, and Nyhan write, "The possibility of contributor influence is especially troubling for elected judges, who have a professional responsibility to serve as dispassionate arbiters of the law but face conflicts of interest when attorney contributors represent client cases before them."

Given these conflicts, can the justices really keep their reelection concerns entirely separate from their courtroom decisions? Maybe they think they can. But the evidence suggests otherwise.

Privately funded judicial elections are widespread

In 38 states, judges have to face voters, which means they have to run campaigns. Only two of those states, New Mexico and West Virginia, provide public funding. (It used to be four, but in addition to North Carolina, Wisconsin also abandoned a public financing system in the last few years.)

Hazelton, Montgomery, and Nyhan are not alone in raising concerns about privately funded judicial elections. Just to pick a few examples:

A 2013 American Constitutional Society report, "Justice at Risk," found that justices who relied on business interests for the majority of their funds were more likely to vote in favor of business interests.

A 2012 Center for American Progress Report, "Big Business Taking Over State Supreme Courts," notes that "donations from corporate America are now overwhelming donations from trial lawyers, labor unions, and groups that support progressive judicial candidates."

A series of joint reports by the Brennan Center, Justice at Stake, and the National Institute for Money in State Politics, have documented the growing millions being spent on state judicial elections. The latest, "The New Politics of Judicial Elections," documented a growing presence of Super PACs running television ads in judicial elections.

A 2006 New York Times investigation into the Ohio Supreme Court found that over a 12-year period, justices voted in favor of their contributors 70 percent of the time. One justice, Republican Terrence O'Donnell, voted in favor of contributors 91 percent of the time.

As retired Supreme Court Justice Sandra Day O'Connor wrote a few years ago, "This crisis of confidence in the judiciary is real and growing. Left unaddressed, the perception that justice is for sale will undermine the rule of law that the courts are supposed to uphold."

This latest paper contributes probably the most methodologically rigorous study to the case against privately funded judicial elections. But the preponderance of the evidence is increasingly clear: Privately funded judicial elections undermine the very idea of judicial impartiality.

Personally, I'd go further and join the rest of the world (except Bolivia) and get rid of all judicial elections entirely. Theoretically, we have legislatures to respond to popular pressures, and courts to buffer the excesses of popular pressure and make sure they conform to constitutional rules. But at the very least, if we do have to hold judicial elections, let's make sure they are not controlled by those lawyers and corporations with direct business before the courts.