The winningest Supreme Court justice
Which Supreme Court justice wins (votes with the majority) the most? Among the sitting justices I suspect it should be Anthony Kennedy, the ideologically median vote on the court. As the swing vote, he should win most of the 5-4 cases. But how does this compare against former justices? To test these ideas I downloaded the justice-centered data from the Supreme Court Database, which has voting records dating back to the FDR administration. For each justice I calculated the proportion of their votes that were in the majority opinion, a kind of ‘batting average’ for votes. I plotted these proportions, listing the justices chronologically by their appointment date. The justices are color-coded by the political party of the president who appointed them, and these presidents are listed on the x-axis.
The winningest justice is indeed Anthony Kennedy, not just on the current court but in the entire dataset. He has won an astounding 91.5% of his cases. But it turns out that the biggest winners aren’t all swing votes. Arthur Goldberg was a solid liberal, replacing the much more conservative Felix Frankfurter. This tipped the balance of the court from a conservative to liberal majority. Goldberg served only a few years, as did Abe Fortas who replaced him. Neither were swing votes, but have high percentages because they served short terms on courts in which they were solidly in the majority. You might also notice that on the current court there are no justices with percentages much below 80%, and this wasn’t true in the past. I discussed the reason for this in a previous post: unanimous decisions. This court often votes either all together or narrowly divided, and in 2013 the court voted unanimously in nearly 2/3 of their cases! If we think that unanimous decisions don’t tell us much about how the court divides, then we can consider at only the cases in which at least one justice dissented. In probability this is called conditioning on the case not being unanimous. Here is the same figure after conditioning.
Everyone’s win percentage drops, with some reaching as low as 55%. And the current court then looks no different than previous courts. It’s also especially obvious in this second figure just how much lower the winning percentages are for the Democratic-appointed justices on the current court are than their Republican-appointed colleagues.
METHODS: The csv formatted justice-centered data from the Supreme Court Database has problems because the case names also contain commas. So I read in the Rdata instead and produced a tab-delimited text file. I then wrote a Python script to calculate the win percentages for each justice and save them to a text file, which I could read back into R and produce the figures.