President Trump shakes hands with Judge Neil M. Gorsuch during an announcement of Gorsuch’s nomination to the Supreme Court at the White House on Jan. 31, 2017. (Jabin Botsford/The Washington Post)

Many have noted that President Trump’s appointees to the federal courts are remarkably less diverse than those selected by President Barak Obama. Only a fifth of Trump’s confirmed judges are women, and 90 percent are white, compared with 40 percent women and just over 60 percent white for Obama. But why does the diversity of the federal bench matter?

Scholars have long noted that diversity affects judicial behavior, especially in cases involving race and gender. Other research suggests African Americans hold the federal courts in higher regard when there are more black judges on the bench.

But judicial diversity also has an indelible effect on the shape of the law. Our new research shows that because judges tend to cite colleagues with similar backgrounds, the demographic and professional backgrounds of federal judges influence the flow of legal ideas throughout the courts.

Here’s how we did our research

When judges write opinions, they typically mention previous, relevant cases to bolster their decisions. These mentions are called citations. To figure out who cites whom across the federal bench, we collected every U.S. Court of Appeals opinion published between 1990 and 2010 that addressed the constitutional prohibition against unreasonable searches and seizures. (Political scientists often rely upon these types of decisions, because judicial behavior on this issue generalizes well to other sorts of cases.)

We then extracted every citation from the opinions and identified the race, gender and professional background of the citing and cited judges. This data enables us to determine whether judges of the same race, gender or similar professional backgrounds are more likely to cite one another’s opinions.

We study citations because they are the bricks and mortar of a court’s decision: They can reassure readers of an opinion’s structural soundness. Most importantly, citations provide a vehicle for spreading legal reasoning across the 12 geographically based federal courts of appeals. So we can use citation counts to measure a judge’s influence across the federal courts. Judges whose opinions are cited more often have a larger influence on the development of legal policy than do judges whose opinions are cited less often.

Race and gender — but also professional background — make a difference

Minority judges promote the work of other judges who share their race or ethnicity more than judges who do not. This practice is similar to a strategy used by women in the Obama administration who practiced “amplification”: purposely and repeatedly crediting the comments of other women in meetings. Interestingly, both female and minority judges are also less likely to be critical of an opinion written by a demographically similar judge.

Judges are particularly likely to look for the work of judges who have similar professional backgrounds. A judge is more likely to cite a judge who attended the same law school, was appointed by the same president or shared a common professional background working as a prosecutor or a judge on another court before joining the appellate court.

For every four citations received by a colleague who shares none of these experiences, a judge will receive five cites from a colleague who shares all of them. Over the course of a judge’s career, these citation patterns add up. There can be substantial discrepancies between judges who have many characteristics in common with a large number of their colleagues and those who do not.

For example, when Obama nominated D.C. federal appellate court Judge Merrick Garland to the Supreme Court in 2016, his top 15 most-cited circuit court opinions had garnered 4,620 citations. As a Harvard-educated former prosecutor, he received citations from a substantial number of his colleagues, building his citation count. Within our data, Garland’s opinions were cited on average roughly four times each. Compare that with the citation rate for a reasonably well-known female judge, Diane Wood, who joined the bench roughly at the same time as Garland but whose law degree is from the University of Texas: Her opinions were cited on average about three times each.

That said, contrary to studies of academic citation patterns, we did not find that white or male judges were less likely to cite the opinions of women or minority judges. Nor are white and male judges more likely to cite their own opinions compared to those by women and minority colleagues. In other words, women and minorities — but not white men — cite one another disproportionately.

Diversifying the federal bench expands the types of judges widely cited by similar colleagues, broadening the range of voices that influence legal development. Trump’s nomination of mainly white and male judges affects not only the outcomes of particular cases, but also the development of the legal rules we live by.

Michael J. Nelson is the Jeffrey L. Hyde and Sharon D. Hyde and Political Science Board of Visitors Early Career Professor in political science at Pennsylvania State University and affiliate faculty at Penn State Law.

Rachael K. Hinkle is an assistant professor in the department of political science at the University at Buffalo, SUNY and a research fellow at the Baldy Center for Law and Social Policy.