As Cass Sunstein points out, the U.S. Supreme Court was mainly a single minded body for its first century and a half with very little dissent. There were occasional dissents and concurrences, probably most notably Justice Harlan’s dissent in Plessy v. Ferguson. Still, the uniformity in decision making made discerning the justices’ distinct views quite difficult. This is what the history of unanimity in Supreme Court decision making looks like based on the percentage of unanimous opinions per term (the case numbers used in this post are derived from the U.S. Supreme Court Database):
Until the drop off in the late 1930’s and early 1940’s, well over 70% of the Court’s opinions were unanimous. What caused the stark drop in unanimity? Many theorize that it had to do with Justices Holmes’ and Brandeis’ more frequent dissents pointing the other justices to the utility of this practice. Also, both the justices and the public saw the Court as a less legitimate branch of the government because the justices were not chosen by the people and so the earlier justices did not want to rock the boat by showing that there were holes in the Court’s analyses which would be pointed out by dissents. With the Court more empowered several decades into the 20th century, this legitimacy crisis was much less of a concern.
With a stronger norm of split decisions, researchers could study the behavior of specific justices in greater detail. An early work on the subject by C. Herman Pritchett was The Roosevelt Court: A Study of Judicial Policies and Values. With separate opinions, ideology measures could be developed that depended on justices voting in different directions across large sets of cases. The quintessential measure that does this is the Martin Quinn Scores (MQ). Due to similar and different voting behaviors, each judge is given a score moving from liberal (negative) to conservative (positive). The measures begin with the 1937 term because there were discernible differences in the justices’ voting behavior from this point forward.
One neat component of the MQ Scores is that a score in one year is scaled so that a score of (+2), for instance, means approximately the same thing as the same score in another year. This is because there are always bringing justices across these Courts or put another way, there is always at least one justice who straddles Court Eras as other justices leave the bench. These justices who bridge other justices allow for comparisons across time.
The variation in scores over time looks like the following:
Here, each color is a different justice, and each dot relates to a justices score in a given term. One point to note is that the justices’ positions were clearly far apart in certain terms like when Rehnquist joined the Court but were also clearly clustered together during most of Justice Vinson’s tenure as Chief Justice.
Martin and Quinn Scores predicated on the justices’ votes underscore when judges vote like one another (close scores) and when they don’t (scores far apart). For the past number of years until Justice Ginsburg died, there was a 5-4 balance in the Court which favored the conservative direction. This led to a high level of 5-4 split votes, especially on issues the public found important. The following graph shows the percentage of 5-4 votes per term since 1936:
The fluctuation on a yearly basis in 5-4 decisions is illuminating. It shows that even with clearly polarized courts, the justices did not always split 5-4 with the same frequency. One reason for this recently is that there was a period after Justice Scalia died where the Court only had eight members.
More recently with the addition of Justice Barrett replacing Justice Ginsburg, the center of the Court shifted to the right so that the conservatives now have a 6-3 majority.
From an ideology score perspective, the transition looks like the following:
The more progressive justices are farther from the center of the graph than some of the conservatives like Roberts and Kavanaugh. The visual shift in the balance of the Court from when Justice Ginsburg passed away and was replaced with Barrett is apparent in the 2020 graph (note that the 2021 term scores are not yet available). This should have impacted the justices’ voting patterns. In particular we should see more 6-3 votes and fewer 5-4 votes from the 2020 term forward.
This is known as a supermajority because even if one conservative justice defects from the majority, the conservatives still can secure 5-4 decisions (think of Roberts concurring in Dobbs even though he wouldn’t have overturned Roe). This may also impact decision making of conservative justices on the fence since even if they vote along with the three other justices in such close cases, it will be in a losing effort.
The current Court is not the only time when the Court has had a 6-3 ideological balance according to the MQ Scores, although this may be one of the strongest 6-3 formations yet.
The first time this orientation was evident, and one of the strongest alliances was during the Hughes Court years. Between the 1936 and 1940 terms the ideological makeup of the Court looks like the following:
The Court had 10 members in the 1937 and 1938 terms with Justice Sutherland retiring and Justice Reed’s appointment. Justice Cardozo passed away right at the end of the 1937 term and was replaced by Justice Frankfurter. Then Justice Brandeis retired in the middle of the 1938 term and was replaced by Justice Douglas. Justice O. Roberts score for the 1937 term was just barely positive at .015 but became more so in 1938 with .352. Along with Justices McReynolds and Butler, Justice O. Roberts makes up the conservative trio for these two years. When Justice Butler passed away during the 1939 term he was replaced with the more liberal Justice Murphy. At this point Chief Justice Hughes pivots to the right and joins Justices Roberts and McReynolds on the right of the Court.
Under the end of Chief Justice Vinson’s tenure and the beginning of Chief Justice Warren’s, the ideological supermajority shifted in the opposite direction.
In 1949 Justices Black and Douglas were the only two justices on the left of the Court and there was a significant distance between those justices and Justice Frankfurter who is the next justice along the ideological map. Frankfurter wavered a little closer to left in the next three terms yet moved back to the left when Justice Warren joined the Court in the 1953 term. Justice Warren then became the third justice on the left in 1954.
Fast forward almost two decades to the Burger Court and another 6-3 formation takes root.
Justices Marshall, Brennan (whose ideological position is covered by Justice Marshall’s in 1972), and Douglas make up the liberal bloc for these three terms. Justice Stewart was close to the ideological center in the 1972 term and pushed a bit farther to the right in 1973 and 1974.
Lastly, an ideologically conservative supermajority was present in the early Rehnquist Court years.
The Court’s left was composed of Justices Blackmun, Stevens and Marshall in 1990. Justice Souter starts out on the conservative end of the Court when he is appointed in 1990 but shifts to the left in the 1991 and 1992 terms. When Justice T. Marshall passes away at the beginning of the 1991 term, he is replaced by Justice Thomas which pushes the pendulum even farther to the right.
That brings us to the modern supermajority of Justices Thomas, J. Roberts, Alito, Gorsuch, Kavanaugh, and Barrett. So how does the strength of the current Court’s makeup compare to those of the past? One way to think of this is through the percentage of 6-3 rulings across each term.
According to these percentages, the 2021 term had the highest percentage of 6-3 decisions across cases that went to oral argument since at least the 1937 Term (which is as far as the MQ Scores go).
Diving a bit deeper into the data though a different story appears. The 6-3 decisions in the above graph do not capture the percentage of 6-3 decisions across ideological lines. Unanimous decisions also do not capture ideological differences, so instead we can focus on the percentage of 6-3 ideological, non-unanimous decisions in these terms. When Justice Barrett joined the Court in the 2020 term the percentage of 6-3 ideological decisions in argued non-unanimous decisions was 27.6%. This jumped up to 29.5% in the 2021 term.
The greatest percentages of 6-3 ideological decisions were in the Hughes Court years though. In two terms, 1937 and 1940, the Court saw 6-3 ideologically split decisions in 35.6% of the Court’s non-unanimous decisions.
The major difference between now and in the past is the staying power of the current ideological bloc. Prior to this term, such supermajorities did not remain stable over more than a couple of terms. The current supermajority is poised to do just that. With Justice Thomas as the oldest justice at 74 years of age, this Court can potentially stay as currently composed for another decade or more. If the Court’s oldest justices, Thomas and Alito are strategic about their retirements, so that their seats are replaced by Republican Presidents, and no justices pass away unexpectedly, this conservative Court could potentially retain its strength much farther in the future.