Why do Supreme Court justices drift toward the center?

Why do Supreme Court justices drift toward the center? This seems to have occured with some Republican-appointed justices over the past few decades and with some Democrat-appointed justices in earlier years. The natural comparison here is to Congress, where I don’t know of any evidence of center-drifting or leftward-drifting of Congressmembers. My law-professor colleague explains the difference as coming from the enviornment of the court. In particular, each case gets discussed and argued (rather than simply voted upon, as with many bills in Congress).

He also points out that judges, by the nature of their job, are exposed to two sides of every issue. Over the years, this could tend to lead to moderation. Unfortunately, most of us do not generally have to seriously consider two sides of every issue. Once again, the comparison to Congress is instructive: Congressmembers are exposed to a lot of lobbyists, who are certainly not divided evenly on issues. (Just pick your favorite issue here: drugs, guns, Israel, . . .) My argument here is not that lobbyists have too much (or not enough) influence but rather that as a judge, you get exposed to arguments in a more structured and balanced way, which might lead to moderation.

Looking at the data

Aleks sent along the following graph of Supreme Court justices, for each year, plotting the proportion of cases for which each judge voted on the conservative side (as coded by Spaeth):


(See here for the bigger version.)

This is a pretty picture–I particularly like the careful use of colors (the original version had some dotted lines but I talked Aleks into just using solid lines)–but one has to be careful in its interpretation. In particular, the graph is telling us about the relative position of justices in any given year, but I wouldn’t trust its implicit claims about changes from year to year, or its long-term trends. The difficulty is that the results shown in this graph depend on the case mix in any given year.

Much of the year-to-year variation in your graph can be attributed to variation in the docket. It’s not clear how to make sense of long-term trends given that the docket is changing over time. In particular, I wouldn’t be surprised if the docket has become more conservative in recent years–as the court has shifted, i’d expect the cases to shift also. Another example is Marshall and Brennan from 1970 to 1990. Do I really believe that they both got more liberal, then both got more conservative, then both more liberal? Well, maybe, but it seems more plausible to me that the docket was changing during these times. This is related to the problem in epidemiology of simultaneously estimating age, period, and cohort effects.

Here’s Kevin Quinn’s estimate of ideal points of Supreme Court justices:


Clearly we need to combine Kevin’s modeling tools with Aleks’s graphics! (Joe, David, Noah, and I fit our own Supreme Court model, but I’m embarrassed to say it didn’t allow judges’ ideologies to move over time. And here’s Simon Jackman’s overview of ideal-point models.)

1 thought on “Why do Supreme Court justices drift toward the center?

  1. Andy's premise probably needs to be tested rather than asserted! I'm not sure either of the figures is evidence of moderation. Andy suggests an interesting conjecture about how justices (or judges more generally) differ from, say, members of Congress. NOMINATE data show that John McCain has become increasingly moderate while Rick Santorum has become increasing extreme.

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