Lawprof Ward Farnsworth has a fascinating article in the October 2005 issue of the Michigan Law Review about the voting patterns of Supreme Court Justices. A draft The final version of the article is available here: Signatures of Ideology: The Case of the Supreme Court's Criminal Docket.

  The article studies voting patterns of individual Justices in nonunanimous cases from the Supreme Court's criminal docket over the last 50 years. The study compares what in theory should be apples and oranges: the rate a Justice voted for the government in constitutional cases and the rate a Justice voted for the government in nonconstitutional cases. Farnsworth's hypothesis, in a nutshell, is that a very high correlation between a Justice's voting pattern in constitutional and statutory cases suggests that ideology is driving decisionmaking. If votes are being cast by applying principled interpretive methods, such as originalism, textualism, etc., there shouldn't be much of a correlation between the two rates for a particular Justice.

  The key finding of the study, as demonstrated in charts in the first few pages of the paper: a very high correlation between voting patterns in constitutional and nonconstitutional cases for almost all of the Justices. As Farnsworth summarizes, "knowing how often a Justice votes for the government in the constitutional cases gives us a very strong sense of how often he votes that way in the cases not involving the Constitution."

  What do these results say about how Justices decide cases? Farnsworth, a former Supreme Court law clerk, offers the following take:

  [T]he point isn't that the decisions are "all politics," or that the Justices always vote their policy preferences. We must remember that these cases we are studying are the non-unanimous ones, and that there are others where the left-most and right-most Justices agree. The better interpretation is that every case provokes competition between a Justice's preferences on the one hand and the legal materials on the other. When the legal materials are very strong, they can produce unanimity despite conflicting preferences. But when the legal materials aren't so strong—when they don't point to a clear answer, and leave room for discretionary judgment—the competition is won by the Justice's underlying preferences and views of the world. Those views of the world are the same regardless of what provision is at stake in a case; that is why there is so much convergence between the results in cases involving different sources of law. Whether a statute or rule or the Constitution is involved, the important question is simply how clearly the Justices think the source of law speaks to the case.

  This is a very interesting and provocative paper. It seems to me that there are some plausible ways in which Justices could be quite principled and still have a high correlation between their votes in constitutional and nonconstitutional cases. Still, this is a very interesting paper, much worth reading. And as an added bonus, the whole thing is only 35 pages long.