First of all, polarization isn’t necessarily a bad thing. Imagine (horror of horrors) a Supreme Court whose members voted en bloc with Ruth Bader Ginsburg. What’s left of the Constitution would disappear in no time.

So, hurrah for polarization when it means that the likes of RBG and her allies on the Court are opposed by — and sometimes defeated by — the likes of Clarence Thomas and his allies on the Court. As of now, Justice Thomas’s allies are (with certainty) Justices Gorsuch and Alito, (sometimes) Chief Justice Roberts, and (one hopes) Justice Kavanaugh, who replaces the too-often compromising Justice Kennedy.

It is therefore my earnest hope that the Court will be, if anything, more polarized than it has been in recent years and decades. How polarized is that? According to Richard L. Hasen, a professor of law at the University of California,

[t]he trend toward presidents choosing more ideologically reliable court appointments began with Democratic president Bill Clinton, following two surprises under Republican president Ronald Reagan. The conservative Reagan appointed perennial “swing” justices Anthony Kennedy and Sandra Day O’Connor (Bartels 2015). Reagan chose Kennedy after the Senate rejected Judge Robert Bork, a more conservative nominee on some key issues (Epstein & Segal 2005).

The days of ideological surprise from appointed justices appear to be over. Today, presidents place “near-exclusive focus on ideological compatibility and reliability” (Bartels 2015, p. 177). Devins & Baum (2017) argue that although both Democratic and Republican presidents have increasingly taken ideology into account in the last four decades, there has been more dramatic movement on the Republican side since the Reagan administration—the first to consider conservative ideology the paramount criteria for selecting nominees. They further contend that the Federalist Society, a private organization of conservative and libertarian lawyers, judges, and activists, has played a central mediating role in the cultivation and choice of Republican judicial nominees (Devins & Baum 2017). This trend has only accelerated in the Trump administration….

Today no one doubts that the Supreme Court is growing more polarized in its decision making. The Court has long been divided into two ideological camps, liberal and conservative, in a bimodal distribution with a center fluctuating in size. While the Court long has been polarized on the basis of ideology [see Clark (2009) on the ways this polarization has been measured over time], it used to boast a larger center and fewer justices at the poles. Ideological polarization has increased in the last 50 years (Gooch 2015)….

For the past few decades and until recently, the Court featured four generally conservative justices, all appointed by Republican presidents; four generally liberal justices, all appointed by Democratic presidents; and swing justice Anthony Kennedy, who often sided with conservatives but sometimes sided with liberals on issues such as same-sex marriage (Bartels 2015, p. 172; Devins & Baum 2017; Hasen 2016)….

… Gone are justices appointed by Democratic presidents who sometimes voted conservatively (Kennedy-appointed Justice Byron White voted against abortion rights) and justices appointed by Republican presidents who sometimes voted liberally (Ford-appointed Justice John Paul Stevens voted in favor of abortion rights) (Bartels 2015, Devins & Baum 2017). Today, each justice’s ideology is better defined and aligned with the political party of the appointing president. Justices are more likely to be ideologically in line with the interests of their nominating president’s party and less likely to drift ideologically (or “evolve”).

Those observations, which will surprise no one who is more than a casual observer of the Court, are from Hasen’s “Polarization and the Judiciary“, Annual Review of Political Science, May 2019 (forthcoming). (Literature cited in parentheses is listed at the end of the paper.)

Neither of the lines in Hasen’s figure 2 resembles, in shape, the results I derived from the Stat Packs at SCOTUSblog, which cover the 1995-2017 terms:

Unanimous decisions include all cases in which there was no dissenting vote, including per curiam decisions, even where the majority opinion was accompanied by one or more concurring opinions. Given the similarity of the two graphs with respect to unanimous opinions, that must be the definition used by Epstein et al. (the source of Hasen’s figure 2).

I am especially struck by the disparity between Hasen’s figure 2 and my graph with respect to the trend (or lack thereof) in decisions with a one-vote margin. (All such decisions during the 1995-2017 terms were by a 5-4 vote.) There is no “polarization paradox”. To the contrary — and as one would expect — there is a strong (though not perfect) negative relationship between unanimous and 5-4 decisions:

Color me unimpressed by Professor Hasen, at least on the evidence of “Polarization and the Judiciary”.

Just how polarized is the Court — or, rather, how polarized has it been recently? Quite polarized.

In “U.S. Supreme Court: Lines of Succession and Ideological Alignment“, I draw on the SCOTUSsblog Stat Packs to summarize the degree of disagreement among the various justices in non-unanimous cases during each of the Court’s past 13 terms. (The use of non-unanimous cases highlights the degree of disagreement among justices, which would be blurred if all cases were included in the analysis.) The statistics yield an index of polarization (P) for each justice, by term:

P = maximum percentage of non-unanimous cases in which a justice disagreed with any other justice during the term

Graphically:

A slight upward trend over the past 13 terms? Perhaps. But trend or no trend, it’s clear that there has been a great deal of polarization among most of the justices. Roberts joined Kennedy in the middle during the past four terms, but there have been (at least) seven highly polarized justices on the Court. In the past two terms, it has been Alito, Gorsuch, and Thomas (on the right) against Breyer, Ginsburg, Kagan, and Sotomayor (on the left).

I relish the hope that Kavanaugh will shore up the right. Now, if Roberts would only revert to his 2005-2013 form….