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« How Ecological Inference Corrupts an Ideology Model | Main | Will Political Science Lose Out to ELS in Law Schools? »

The History of Bivariate Ecological Regression in Judicial Politics

[Version 1.5]*

It is challenging to commence an organized analysis of the problems inherent in Segal and Spaeth’s Supreme Court decision-making literature. Indeed, one could enter this discussion from a number of areas. But rather than beginning where others have, I want to start with an entirely original point that is relevant to my own labors in this field: political science’s attempt to create a “bivariate ideology model.”

What is a bivariate ideology model? It is simply a mathematical model of decision making that estimates the relationship of a lone, single variable -- justice ideology -- upon the choices justices make in Supreme Court cases (votes on the merits). Five or ten years ago, it was common to find political scientists appealing to these models as “proof” of the primacy of politics over “law” and the exposure of a popular “mythology” surrounding Supreme Court judging.[1]  The creators of these models apparently still put forth these views.[2]  The truth, however, is that the bivariate ideology models created by political scientists never established the conclusions commonly attributed to them. For now, however, let us begin with an overview of the basic nature of these models as well as their history.

Any discussion of the history of the bivariate ideology model  in attitudinal literature must begin with Segal and Cover’s (1989) landmark article. It was this article that was said to provide the first systematic explanation of the voting behavior in civil liberties cases using an independent variable not derived from the justices’ votes (557). To accomplish this goal, attitudinal researchers created an empirical index called “Segal/Cover scores,” which were derived from the content of newspaper editorials appearing during justice confirmation hearings. The scholars coded editorials describing nominees as liberal or conservative and scaled the results, creating what in essence is a measure of each justice’s reputation for political bias at the time of his or her confirmation. Importantly, the scholars then decided to regress the scores not against the actual civil-liberties voting data that existed, but against a set of summary percentages derived from that data. In short, they regressed Segal/Cover scores against the justices’ percent-liberal ratings. The results of the regression showed a rather high correlation coefficient of 0.80 (561). Other scholars in 1995 joined in this example and created an updated analysis that again offered a robust correlation of 0.80 (Segal et. al., 1995).

Based upon these studies, many political scientists began concluding that the attitudinal model was now an empirically dominant explanation of justice voting behavior, and that justice ideology governed the bulk of the choices made in civil liberties cases.[3]  So popular did bivariate ecological regression become among judicial politics scholars that, even today, it continues to appear in the literature. Jeff Segal’s (2005) work, in fact, analyzes the Rehnquist Court with a bivariate ecological model that focuses exclusively on the 14 justices who served under William Rehnquist’s tenure as chief justice. He reports a correlation of 0.70 for civil liberties cases and 0.72 for the entire docket. There is also a bivariate ecological model that appears in Epstein, Knight and Martin’s recent work on civil rights voting (2004, 181; Figure 10.3). (Even the New York Times recently printed aggregated scatter plots).[4] Perhaps the best statement, however, of what political scientists thought they had proven with bivariate ecological regression can be found in Epstein and Knight’s (1998) work, which replicates the now-famous scatter plot and declares why it is relevant (35,36):

When it turned out that [Segal and Spaeth] could explain more than 60 percent of the variation in civil liberties votes based solely on the justices’ policy preferences, the researchers concluded that justices come to the bench with a set of policy preferences, which they pursue through their votes, at least in civil liberties cases.

Over the last five to ten years, many political scientists offered similar pronouncements.[5]  Of particular interest is Brisbin’s (1996) assessment of the evidence that appeared in political science’s top journal. It declared that the case in favor of Segal and Spaeth was so cogent that further study of the issue should actually cease (1004). It also gave to the APSR the following observation (1011):

If the fiction of a Court of law and not politics, like the tale of a fire breathing dragon, is now dead, why belabor it through further study? Perhaps it is because the dragon is dead, but like most dead reptiles, he is still twitching. So, for good measure, it is necessary to drive lances into him again and again and then draw and quarter him so that the heresy of a legal model of Supreme Court decision making cannot be regenerated.[6]

To understand why these bivariate ideology models are problematic, one must first understand the data that comprise them. The source is a large, publicly-available resource known as the Supreme Court Data Base, which contains voting and case data for every justice who served on the Court from 1946 through 2004.[7]  The format of the variable that observes the ideological choices of the justices is a simple binary entry coded with a "1" or “2” if the vote is liberal or not (Spaeth 1999, 69-72, 92). The total number of civil-liberties votes accounted for by this resources is over 31,049, covering 58 continuous years of Court activity by 32 justices. By aggregating this data into a handful of percentages and using the same as a dependent variable in a regression model, political scientists introduced ecological inference into their empirical analysis. This appears to have created confusion and exaggeration in the interpretation of model results. The truth is that Segal and Spaeth’s bivariate ideology model only accounts for about one-third of the level of explanation the researchers proclaimed. This is still a reasonable model, of course, but it is nowhere near the level of deconstruction many political scientists had proclaimed – and, in fact, supports a much more limited critique of the role that ideology plays in judging.  

[1]. Segal and Spaeth, The Attitudinal Model Revisited, 1, 8, 10 and 26-27.

[2]. Segal, Jeffrey. 2005. “The Rehnquist Court” Law & Courts. 15 (Spring): 14-17.

[3] Evidence of this is found in the following declarations: (1) “A prominent view, if not the prominent, view of U.S. Supreme Court decision making is the attitudinal model. It supposes that the ideological values of jurists provide the best predictors of their votes …” (Segal, et. al. 1995); (2). “[xx-get this quote]” (Peretti 1999, 105-111); (3) “Spaeth’s conclusion about the value of the attitudinal model is one echoed by many scholars of the judicial process, and not just those working in the area of decision making. ... Justices do not decide a priori to protect minority rights or to legitimate the ruling regime. Rather, they base their votes on their political ideologies, with a consequence being that liberal justices tend to protect minority interests, while conservative ones tend to legitimate the ruling regime” (Epstein 1995,  249-250); (4) “... attitude theory is still regarded by most judicial behavioralists as the most elegant and persuasive model for predicting appellate judge behavior” (Carp and Stidham 2002, 351); (5) “Among many political scientists, aspects of the attitudinal model have become a virtual truism” (Cross 1997, 251, 265); (6) “Today, few political scientists would dispute that, within their discipline, the leading approach to adjudication is the ‘attitudinal model,’ which hypothesizes that Supreme Court justices vote their political preferences or ideologies” (Feldman 2005, 89-90); and (7) “Indeed, these days it is difficult to argue credibly that the model utterly fails to perform its primary task. The evidence in support of its one observable implication – namely, that the policy preferences of the justices help predict their merits votes – is overwhelmingly in its favor” (Epstein 2003). See also, Gillman (2001, 465-466) (asserting that judicial behavioralist scholars believe that “law has almost no influence on the Justices” of the Supreme Court) and Brisbin (1996) (quoted, supra, p. 5).  But perhaps what says it best is Segal and Spaeth’s now famous (2002, 86; 1993, 65) summation of their research, “Rehnquist votes the way he does because he is extremely conservative; Marshall voted the way he did because he was extremely liberal.”

[4] See The New York Times, January 6, 2006.

[5] See Notes 12 and 15.

[6] The author continues later in the article: "If additional empirical analysis is coupled with a politically conscious interpretation of legal texts, judicial research could not just slay any claims for principled, legal models of Supreme Court decisions making, it could slay any prescriptive arguments that endeavor to separate legal decisions from politics. Using multiple levels of analysis deciphering the components of judicial attitudes, judicial scholars could in effect deconstruct any claim that American law is or can be a morally principled effort to write down the rules used to discipline political pathologies" (1014). (Although it is true that some of this exaggerated praise arose not only from the results of ecological regression, but from other models that Segal and Spaeth were producing, it is also true that if you take away the bivariate ecological model, not enough remains in the other models to support such an observation -- at least not empirically).

[7] There are several data sets available. See the Ulmer Project at:

* copied paragraphs from manuscript version

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