In last week’s Senate confirmation hearings for Judge Amy Coney Barrett’s nomination to the Supreme Court, her opening statement included the comment, “I clerked for Justice Scalia more than 20 years ago, but the lessons still resonate. … His judicial philosophy is mine too.” That comment drew praise from Republicans while annoying Democrats, still angry that the GOP-controlled Senate wouldn’t hold hearings for President Barack Obama’s 2016 nominee to replace Scalia, Merrick Garland.

So what exactly is Barrett’s judicial ideology, compared to that of other federal judges? And what would be the ideological effects of adding Barrett to the Supreme Court?

Political scientists have used voting records to measure the ideological leanings of presidents, members of Congress, Supreme Court justices, state legislators, and state supreme court justices. These measures, called “ideal points,” allow researchers to compare the ideologies of state and federal officials across branches and over time, on a left-right ideological spectrum.

However, to date no one has created ideal point measures for judges on the federal courts of appeals that are both based on the judges’ voting records and comparable across the courts of appeal and the Supreme Court. That’s made systematic examination of Supreme Court nominees nearly impossible, with scholars, journalists and senators relying instead on indirect proxies or a nominee’s positions on individual issues like the Affordable Care Act, reproductive rights and hypothetical cases.

Our new data on federal appellate courts can tell us more about Supreme Court nominees

To address this lack of data, our team of social scientists and data scientists collected an original data set of all decisions made by each of the 13 U.S. Courts of Appeals — going back to when each court was created. Here, we’ll focus on the approximately 1.2 million cases decided between 1980 and 2020. We combined these cases with data on the U.S. Supreme Court for the same time period and used statistical methods to estimate ideal points for each judge in the data. As different courts rarely hear the same cases, we couldn’t easily compare judges’ scores across courts. To get around that problem, we used the estimated ideology of each judge’s appointing president to convert, or “scale,” the ideal points so that they can be compared across courts. That’s similar to taking raw counts of correct answers on a standardized test (such as the SAT) and converting them into a traditional “scaled score” (e.g., between 200 and 800) in order to compare scores across sections of the test.

How does Barrett compare to Supreme Court justices?

The graph below plots two dimensions of these ideology measures for the 517 judges in our data. The first dimension, along the x axis, charts where the judges sit on the traditional left-right political division. The second dimension, on the y axis, charts where they came down on highly salient cases along some social and civil rights issues. The blue circles represent judges appointed by Democratic presidents; red circles indicate Republican appointees.

Interestingly, Barrett’s ideology score on both dimensions is closest to that of her mentor, the late Justice Antonin Scalia. If our estimate accurately reflects how she will vote on the Supreme Court, she would be the second most conservative member of the Court, behind only Justice Clarence Thomas.

We also estimated how much the Court’s ideology would shift with this new appointment by examining the ideological distance between Barrett and the justice she would replace. Our work suggests that, in taking the seat of the late Justice Ruth Bader Ginsburg, Barrett’s appointment would create the largest ideological shift caused by a single seat’s replacement during our 40-year period. In fact, Barrett would shift the high court rightward even more than the appointment of Thomas, a conservative appeals court judge, did when he replaced the liberal stalwart, Justice Thurgood Marshall.

Will Barrett’s judicial ideology change after she is appointed?

But that assumes that Barrett’s decisions would remain ideologically similar to her decisions on the Court of Appeals for the 7th Circuit. Do appeals court judges adjust their behavior once they join the Supreme Court? Several scholars have suggested that Supreme Court Justices change their ideological profiles after reaching the high court. Indeed, one study found that the vast majority of justices shift substantially after their appointment.

That’s what we find as well by comparing the scores of judges when they served on a U.S. court of appeals (if they did) to their scores after they were seated on the Supreme Court. In nearly every instance, judges shifted on one or both measures. Scalia remained relatively constant on his basic “left-right” score, but became more conservative on high-profile cases.

Other judges, like Anthony M. Kennedy, became more moderate on the left-right dimension and more conservative position in the more salient cases, when comparing his Supreme Court decisions to those when he sat on the 9th Circuit. As a Supreme Court justice, Trump appointee Neil M. Gorsuch now votes in a way that is slightly less conservative on the left-right dimension than he did when on the 10th Circuit. In other words, while it’s hard to say how she might change, we can say that it’s highly likely Barrett will shift after her appointment.

What does this mean for Barrett’s confirmation?

Of course, a Justice Barrett could resist the general tendency to shift how she votes to follow the public’s mood, as some research (but not all) suggests happens for many. Or, since the liberal bloc no longer has the necessary four votes to grant certiorari, the Supreme Court could be hearing more conservative cases. If Judge Barrett does become Justice Barrett, as appears likely, we will soon find out.

Jason Windett is an associate professor of political science and public administration, associate professor of public policy, and an affiliate of the School of Data Science at the University of North Carolina at Charlotte.

Jeffrey J. Harden is an Andrew J. McKenna Family Associate Professor of Political Science and Concurrent Associate Professor of Applied and Computational Mathematics and Statistics at the University of Notre Dame.

Morgan L.W. Hazelton (@HazeltonPhDJD) is an assistant professor in political science and law (by courtesy) at Saint Louis University.

Matthew E.K. Hall is a professor of political science and concurrent professor of law at the University of Notre Dame.

This research was supported by the National Science Foundation’s Convergence Accelerator, Award OIA-1937033, “Building the Federalism Data and Advanced Statistics Hub.”