Why do judges do what they do? It is easy to identify two different answers. The first emphasizes the law. The second emphasizes politics.

Some people say that because they are merely interpreting the law, judges are altogether different from political actors. In his confirmation hearing, Chief Justice Roberts famously described judges as neutral umpires, calling balls and strikes. In 2001, Justice Sotomayor declared that “I would hope that a wise Latina woman with the richness of her experiences would more often than not reach a better conclusion than a white male who hasn’t lived that life,” but at her confirmation hearing in 2009, she said that “judges must apply the law and not make the law. Whether I’ve agreed with a party or not, found them sympathetic or not, in every case I have decided, I have done what the law requires.” Many people are drawn to this general account—if not for judges in general, at least for the judges with whom they agree. Some of Justice Scalia’s admirers lament that judges do not always do “what the law requires,” but they think that Justice Scalia does exactly that. Some people describe this perspective as “legalist.”

The opposing view is that judges are inevitably political actors, and hence their decisions are ultimately based on their ideological convictions. Sure, judges hide behind the law, and they purport to be speaking for it, but we shouldn’t be fooled. If Justice Scalia disagrees with Justice Sotomayor in an affirmative action case or a campaign finance case, the explanation is not that one of them follows the law while the other ignores it, but that conservatives and liberals disagree about affirmative action and campaign finance regulation. In honor of the legal realist movement of the 1930s, which emphasized the role of the judge’s predispositions, we might describe this view as “realist.”

Having written articles with titles such as “A Political Court,” Richard Posner, himself a judge, has long tended to favor the realist view. But as a dedicated empiricist, he knows that the underlying debate is an empirical one, which should be answered by reference to data. In baseball, political campaigns, and many other areas, people are now going beyond intuitions, anecdotes, impressions, and dogmas to incorporate careful statistical analysis. So can we play moneyball with judges? Can the judiciary find its Nate Silver?

A lot of people think so. In recent decades, researchers have been counting and cataloging many thousands of judicial votes. Armed with statistical techniques, they have tested a variety of hypotheses about what judges do and why they do it. Posner likes this enterprise, and he is now a part of it. He has teamed up with Lee Epstein (a political scientist) and William Landes (an economist) to provide a comprehensive, numbers-filled assessment of judicial voting patterns.

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