The Supreme Court’s history is sorely lacking in diversity, presidents and Senates prefer nominees who are ideologically like them, and other findings about SCOTUS

Ahhh, the Supreme Court of the United States. Key player in our federal government’s system of checks and balances. The Highest Court in the Land. Nine superheroes cloaked in black. Responsible for so many critical milestones in our county’s history, from denying citizenship to African American slaves (Dred Scott v. Sandford, 1857) to upholding “separate but equal” segregation laws (Plessy vs. Ferguson, 1896) to upholding Japanese Americans’ internment during World War II (Korematsu v. United States, 1944) to banning segregation in public schools (Brown v. Board of Education, 1954) to affirming women’s right to an abortion during the first two trimesters (Roe v. Wade, 1973) to letting corporations and unions spend unlimited amounts in elections (Citizens United v. Federal Election Commission, 2010) to legalizing same-sex marriage in all 50 states (Obergefell v. Hodges, 2015). Embodiment of our changing moods as a country. Embodiment of our partisan divisions.

With the stakes immensely raised through Trump’s election and impending SCOTUS nominations, I was eager to learn more about our highest court of law. What types of people served on it throughout history? Who do presidents tend to nominate? Who does the Senate tend to confirm?

But first, some foundations about the Supreme Court. You probably already learned all this in your high school Civics class, but here’s a refresher just in case. Established through Article III of the U.S. Constitution in 1789, SCOTUS leads one of our three branches of federal government. Presidents nominate justices. The Senate confirms the nominations through simple majority (meaning: more people say Aye than Nay).

The Court consists of nine justices — one chief justice and eight associate justices. They’re ridiculously qualified people who serve for life. But occasionally there are sad times when not all seats are full. Now is one of those times. Justice Antonin Scalia passed away in February 2016 and the Republican-controlled Senate has refused to act on Obama’s nomination, Merrick Garland. So while the rest of 2016 moved along (very, very, very slowly, it would seem), the Supreme Court has been deadlocked and unable to make decisions on things like the deportation of illegal immigrants and whether public-sector unions can collect fees from even employees who chose not to join. Fun. Times.

But who are these Supremes? Who tends to get nominated and confirmed? Using data consolidated by the Washington University in St. Louis, I set out to investigate the Court. For the purposes of this analysis, I did not include recess appointments, which are Supreme Court appointments made by the president while Congress is in recess (anyone appointed this way must still be confirmed by the Senate by the end of Congress’ next session in order to keep his/her seat).


The faces of justice have been very lacking in diversity, but it’s starting to improve (sort of)

Here are the Supreme Court justices we have today:

  • John Roberts (Chief Justice): nominated by George W. Bush, on SCOTUS since 2005
  • Anthony Kennedy: nominated by Reagan, on SCOTUS since 1988
  • Clarence Thomas: nominated by George H.W. Bush, on SCOTUS since 1991
  • Ruth Bader Ginsburg: nominated by Clinton, on SCOTUS since 1993
  • Stephen Breyer: nominated by Clinton, on SCOTUS since 1994
  • Samuel Alito: nominated by George W. Bush, on SCOTUS since 2006
  • Sonia Sotomayor: nominated by Obama, on SCOTUS since 2009
  • Elena Kagan: nominated by Obama, on SCOTUS since 2010

Three of the eight current justices are women — not bad, right? Turns out that’s a very rare and very recent phenomenon.

Like our other branches of government, the Supreme Court has been sorely lacking in diversity. Its history is very male, very white, very rich, and very Ivy League. The good news is this is starting to change, albeit slowly.

Women are a very new presence on the Supreme Court.

Note: for this chart, “% of justices who are women” refers to people who BECAME justices in the time period specified, so for example the 10% refers to the percentage of women among people who became Supreme Court justices in the 1950–2000 period. This applies to all other charts throughout this post as well.

There were literally no women nominated for SCOTUS until Sandra Day O’Connor, nominated by Reagan in 1981 and successfully confirmed by the Senate. Seriously, zero women until just 35 years ago. The gender gap is so wide in the Supreme Court’s history that there have literally been more than 5 times as many nominees named John as female nominees, and almost 3 times as many justices named John as female justices.

Things have been improving in more recent years, with 43% of the nominees since 2000 being women. Womankind is finally starting to beat the John’s of the country as far as the Supreme Court is concerned.

Red, white, and blue? SCOTUS has really just been white.

The record isn’t much matter along racial lines. Before Lyndon Johnson nominated Thurgood Marshall in 1967, 100% of nominees were white. Even today, the percentage of white nominees and justices outstrips the proportion of the U.S. population that’s non-Hispanic white (63%), although the progress toward racial diversity is directionally promising.

Justices come from wealthy backgrounds, although this has been changing.

This is probably the most encouraging demographic trend so far: over time, the proportion of SCOTUS nominees and justices who were raised in lower class, lower-middle class, or middle class families has increased steadily. There has been a dip in progress since 2000, though, although it’s probably too early to tell what this half century will look like in terms of SCOTUS’ socioeconomic class makeup.

SCOTUS is very Ivy and actually getting increasingly so.

While SCOTUS’ socioeconomic backgrounds seem to be diversifying, its law school composition is going in the opposite direction. The percentage of nominees and justices who attended an Ivy League law school is rapidly increasing over time. Harvard, Yale, and Columbia alone actually account for 58% of all justices since 1950. This is perhaps not shocking, given our labor market in general is demanding ever more elaborate credentials, although this dominance by Ivy League trainees does raise questions about the perspectives represented on our highest court.

Age has held steady over time.

The average age of Supreme Court nominees and justices have stayed pretty consistent throughout history, at the 50s. Additional analysis is needed to explore why this is the case, especially since there are no age requirements to become a Supreme Court justice and the country is aging overall.

We still have a very long way to go when it comes to diversity among the justices, although recent changes seem encouraging. With these demographic trends in mind, I dug deeper into who presidents tend to nominate and who Senates tend to confirm.

Presidents tend to nominate people who hold similar beliefs as them

Probably not shockingly, presidents like to nominate people who are ideologically similar to them.

There are some exceptions, but presidents generally nominate from within their own party. As the chart above shows, Democrats nominate mostly Democrats; Republican nominate mostly Republicans. Federalists nominate Federalists; Whigs nominate Whigs. (If you’re wondering, wait what are Federalists and Whigs again? — here’s a quick refresher. Federalists and Whigs were U.S. political parties active primarily in the early-mid 1800s. To grossly generalize, the Federalists wanted a strong national government and fiscally sound policies, whereas the Whigs championed Congress’ rather than the president’s dominance as well as government promotion of economic growth.) It’s also interesting to see from this chart above that Democratic presidents and Republicans presidents have made about the same total number of nominations.

Presidents’ tendency to nominate people like themselves is actually stronger than just party alignment. Presidents not only like to nominate from within their own party; they like to nominate people with the same level of liberalism/conservatism. This graph below shows nominating presidents’ ideology along the X axis as measured via the Common Space DW-NOMINATE score, which was developed in the early 1980s by political scientists Keith Poole and Howard Rosenthal and often used to measure where political figures fall along the ideological spectrum (with 1 being most conservative and -1 being most liberal). The Y axis shows the nominees’ ideology via the Segal-Cover score, developed by Jeffrey Segal and Albert Cover explicitly to measure the ideology of Supreme Court nominees (0 means most conservative and 1 means most liberal). In case you’re wondering, the circles in the chart represent nominees who went on to become justices and the x’s represent nominees who didn’t.

Correlation between the ideology of presidents and the ideology of people they nominate for SCOTUS. For an interactive version of this chart, see here: https://public.tableau.com/views/SCOTUS/presidentideologynomineeideology?:embed=y&:display_count=yes.

This correlation between the president’s and nominee’s ideology is strongly statistical significant (p-value < 0.0001, for you statisticians). The more liberal a president is, the more liberal his/her nominees tend to be (I know only the “his” pronoun makes sense right now when talking about U.S. presidents but I’m going to continue saying “his/her” out of principle). Apparently presidents want justices who are just like them, not only in party but also in ideology.

The Senate also tends to confirm nominees who are ideologically similar to them

Turns out Senators, too, want justices who are just like them. As mentioned earlier, for a nominee to successfully take a seat on the Supreme Court, the Senate needs to confirm the nomination via simple majority, meaning more Senators need to be in favor than opposed. Historically, a nominee’s chances of getting confirmed are actually not bad. 72% of all nominees since our country’s founding have been successfully confirmed, although this has varied widely over time:

What factors actually affect the Senate’s likelihood of confirming a nominee?

Qualifications matter little

One factor that I thought would play a role, but turned out not to, was how qualified the nominee was for the job. Sad? I know. This chart below shows the confirmation % of nominees across different levels of qualification. The qualification scores are from Segal-Cover, which not only evaluates Supreme Court nominees’ ideology (as mentioned above) but also how qualified they are to be on SCOTUS. 0 means least qualified and 1 means most qualified.

This shows that nominees in the middle and at the very high end of the qualification scale have been confirmed at high rates, although the relationship between the two axes doesn’t appear particularly clear.

Party matters little

Surprisingly, party allegiance doesn’t seem to matter much either when it comes to which nominees the Senate confirms. This table below shows what percentage of nominees made it successfully to SCOTUS, cut by the nominee’s party as well as which party controlled Senate at the time of nomination:

There’s a lot going on here so let’s break it down.

  • Republican nominees have had slightly more luck than Democratic nominees (79% success rate versus 69% success rate), but the difference is small.
  • Democrat-controlled Senates and Republican-controlled Senates have confirmed nominees at the same rate (77%).
  • Republican-controlled Senates are just as likely (77% likelihood) to confirm Democratic nominees as Republican nominees. Democrat-controlled Senates have actually confirmed Republican nominees at a higher rate (84%) than Democratic nominees (77%) although the difference is minor.
  • Whigs have issues making friends. Whig nominees get confirmed at very low rates (17%), and Whig-controlled Senates confirm nominees — including those from their own party — at very low rates (11% overall). I wonder if not having any friends is one of the reasons they’re no longer around as a political party.
  • Note: Independent nominees look like they have such a high confirmation rate (100%) because there’s only been one Independent nominee in history: Felix Frankfurter, nominated by FDR in 1939 and successfully confirmed.

So, interestingly, party affinity doesn’t actually seem to play a big role in who gets confirmed.

Ideology matters a lot

The main factor that appears strongly correlated with confirmation results turns out to be ideological alignment between the Senate and the nominee. Below the Y-axis represents the nominee’s ideology measured via the Common Space DW-NOMINATE score mentioned earlier. The X-axis represents the ideology of the median Senator at the time of nomination, also measured via the Common Space DW-NOMINATE score, where 0 means most conservative and 1 means most liberal. Here’s what this looks like if we focused only on the nominees who made it successfully onto SCOTUS:

Correlation between the ideology of the median Senate member and the ideology of the nominee, for nominees who were successfully confirmed. For an interactive version of this chart, see here: https://public.tableau.com/views/SCOTUS/senateideologynomineeideologysuccessful?:embed=y&:display_count=yes.

What we see here is that in situations where nominees ended up being successfully confirmed, there was a clear correlation (statistically significant, p-value: 0.026) between the Senate’s median ideology and the nominee’s ideology. In other words, where there’s a directional match between the nominee’s and the Senate’s level of liberalism/conservatism, the nominee tends to make it through the confirmation process.

Contrast this with nominees who did not end up making it onto SCOTUS:

(Lack of) correlation between the ideology of the median Senate member and the ideology of unsuccessful nominees. For an interactive version of this chart, see here: https://public.tableau.com/views/SCOTUS/senateideologynomineeideologyunsuccessful?:embed=y&:display_count=yes.

Granted, the sample size is pretty small here because DW-NOMINATE data is unavailable for some nominees and some Senates, but we can see there’s no real trend here (FYI the p-value here is 0.8 — not statistically significant). In situations where the nominee did not make it onto the Supreme Court, there was little correlation between Senate and nominee ideology.

So funnily, it turns out party alignment and ideological alignment are two pretty different things. Whereas party overlaps seem to have limited relation to a nomination’s likelihood of confirmation, ideological affinity seems to play a big role.

Justices’ votes as part of SCOTUS align with their pre-SCOTUS ideology

Are Supreme Court justices’ behaviors after they join SCOTUS similar to their behaviors prior? The story that usually pops to mind for me is Earl Warren. Eisenhower appointed him in 1953 believing that Warren (a Republican) would uphold conservative values on the Court, only to have Earl Warren turn out to lead some of the most liberal decisions in Supreme Court history, including banning public school segregation (Brown v. Board of Education, 1954) and outlawing mandatory school prayer (Engel v. Vitale, 1962).

Turns out Warren was quite the exception to the norm. For the most part, justices’ voting behaviors as part of SCOTUS match up well with their pre-SCOTUS ideology.

Correlation between the pre-SCOTUS ideology of the nominee and the percentage of their votes that are liberal after joining SCOTUS. For an interactive version of this chart, see here: https://public.tableau.com/views/SCOTUS/nomineeideologyvoting?:embed=y&:display_count=yes.

Above, the X-axis represents a nominee’s ideology via the Segal-Cover score, which looks at someone’s ideology prior to their confirmation (0 = most conservative; 1 = most liberal). The Y-axis is what percentage of someone’s votes as a Supreme Court justice is liberal-leaning, as calculated by researchers at the University of Washington in St. Louis. The relationship between these two factors is statistically significant (p-value: 0.0003). People who were deemed more liberal tend to vote more liberal; people who were deemed more conservative tend to vote more conservative. So in case you’re wishing that Trump’s nominee will unexpectedly start voting liberal once he or she gets to the Supreme Court — well, it’s not impossible, but the chances are small.


Takeaways, recommendations, learnings

So what’s the moral of the story here? To summarize the main findings -

  • Throughout history the Supreme Court’s composition has been very male, very white, very wealthy, and very Ivy League. Most of this is starting to change, with the exception of Ivy League which is accounting for an increasingly higher share of justices’ legal education.
  • Presidents have historically nominated people who share their own beliefs — not only people who have the same party loyalties but who are also close to where they are on the ideological spectrum.
  • Senates have also historically confirmed people who share their own beliefs. Party affinity surprisingly doesn’t seem to matter much, but ideological closeness does. The qualification of the nominee also doesn’t seem to have a big impact on confirmation outcomes.
  • Justices’ voting behaviors tend to align closely with their pre-SCOTUS ideology. Earl Warren was an anomaly.

My primary recommendation from this analysis is for the Ivy League (because I know they read my Medium posts…) -

  • To the Ivy League: you’re educating the vast majority of our past, current, and likely future Supreme Court justices. Please make sure the education you provide is more balanced in perspective than the historical demographics of SCOTUS.

And as always, since I am still relatively new to this level of data analytics, let me wrap up with what I learned from the process of doing this project.

  • Choose your weapon wisely. I did this entire analysis in Excel and Tableau, and did not use SQL this time around. Different tools are good for different things, and you can gain a lot of efficiency by tailoring tool to problem as strategically as possible. I think getting better at that was what allowed me to complete this particular analysis in record time, although I think I could have moved even faster if I used SQL for a few of the questions I was trying to answer.
  • I would benefit from learning more data science and statistics. My recent data projects have involved a lot of correlation analyses, and I think I can extract even deeper insights if I had a better understanding of the statistical nuances underlying the correlations.
  • The quality of the data is incredibly important. For this project, researchers at the University of Washington in St. Louis already did the lion share of the data collection work by bringing together nominees’ demographic information. They also had to consolidate political ideology scores from two different systems (Segal-Cover and DW-NOMINATE) because researchers before them had created and applied different types of scores to different political figures. While having to use two different scoring systems side by side presents its own limitations, I am grateful to have had this pre-consolidated dataset to work from!
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