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The Effect of Ideology on Supreme Court Judging Over Time

Empirical scholars of the United States Supreme Court, Jeffrey Segal and Harold Spaeth have long contended that Supreme Court decisions are based primarily upon the ideological beliefs of the justices, and that ideology alone accounts for over 60% of the total voting variance on the Court. However, recent scholarship demonstrates that this conclusion is only made possible through ecological inference. In the bivariate regression that purported to establish their hallmark conclusions, Segal and Spaeth aggregated their voting data into percentages. The result was exaggerated findings. Recent scholarship has corrected Segal and Spaeth’s mistake by modeling justice votes using a logistic regression that does not manipulate the dependent variable before analysis is performed. The new findings demonstrate that Segal and Spaeth’s model loses about two-thirds of its explanatory value. Ideology, therefore, is not as dominant of a force upon the Court as attitudinal modelers previously thought. Still, however, it remains an important variable in the judging equation. It most certainly is a statistical predictor. However, this paper adds two important contributions to above findings: (1) the role that ideology plays upon the Court is not stable across time and appears to be trending downward; and (2) Segal Cover scores are a relatively poor explanation of voting behavior and at times are a complete failure. In so doing, the findings of this paper encourage political scientists to reconsider the way that they conceptualize the force of ideology in the judicial mind. To the extent that ideology is measured correctly by empirical researchers, it is a fluctuating rather that defining force upon the Court. That is, it is sometimes a significant and dominant presence in voting; at other times, it is marginalized and rendered less significant. Finally, if researchers wish to model ideology most effectively, all available empirical evidence suggests that Segal Cover scores should be abandoned in favor of career liberal ratings. Quite simply, these scores approximate judicial choice more meaningfully than any other set of ideological values currently available.

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