Ideological leanings of U.S. Supreme Court justices

The U.S. Supreme Court is entrusted with resolving disputes about how the United States Constitution and other federal laws should be applied to cases that have been appealed from lower courts. The justices base their decisions on their interpretation of both legal doctrine and the precedential application of laws in the past. In most cases, interpreting the law is relatively clear-cut and the justices decide unanimously without dissent. However, in more complicated or controversial cases, the Court is often divided.

It has long been commonly assumed that the votes of Supreme Court justices reflect their jurisprudential philosophies as well as their ideological leanings, personal attitudes, values, political philosophies, or policy preferences. A growing body of academic research has confirmed this understanding: scholars have found that the justices largely vote in consonance with their perceived values. Analysts have used a variety of methods to deduce the specific perspective of each justice over time.

Researchers have carefully analyzed the judicial rulings of the Supreme Court—the votes and written opinions of the justices—as well as their upbringing, their political party affiliation, their speeches, editorials written about them at the time of their Senate confirmation, and the political climate in which they are appointed, confirmed, and work. From this data, scholars have inferred the ideological leanings of each justice and how the justices are likely to vote on upcoming cases.

Using statistical analysis of Supreme Court votes, scholars found that an inferred value representing a Justice's ideological preference on a simple conservative–liberal scale is sufficient to predict a large number of that justice's votes. Subsequently, using increasingly sophisticated statistical analysis, researchers have found that the policy preferences of many justices shift over time. The ideological leanings of justices (and the drift over time) can be seen clearly in the research results of two sets of scholars using somewhat different models:

Andrew D. Martin and Kevin M. Quinn have employed Markov chain Monte Carlo methods to fit a Bayesian measurement model of ideal points (policy preferences on a one-dimensional scale) for all the justices based on the votes in every contested Supreme Court case since 1937. The graph below shows the results of their analysis: the ideological leaning of each justice from the term beginning in October 1937 to the term that began in October 2015. Note that the scale and zero point are arbitrary—only the relative distance of the lines is important. Each unique color represents a particular Supreme Court seat, which makes the transitions from retiring justices to newly appointed justices easier to follow. The black lines represent the leanings of the Chief Justices. The yellow line represents the estimated location of the median justice—who, as Duncan Black’s median voter theorem posits, is often the swing vote in closely divided decisions.

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