No one seriously questions that ideological bias influences judicial decisions. But, as with most things, the interesting question is not whether but how much? Using a modified version of the Segal-Cover ideology scores that have proven highly predictive in civil rights cases, Nancy Staudt, Lee Epstein, and Peter Wiedenbeck analyzed every U.S. Supreme Court case decided between 1940 and 2005 that involves an interpretation of the Internal Revenue Code. Here are the Segal-Cover scores of the Supreme Court justices since 1940.

When the Court is extremely liberal, the probability that the government prevails is 83%; whereas when the Court is at its most conservative, the probability that the government will prevail falls to 47%. A moderate court (i.e. a Court with the median coded as 0) will produce a government win roughly 63% of the time, which is the mean percentage of government wins in our database of all income tax cases. These statistics offer an explanation for why the government won more often in the 1950s and is currently losing corporate tax cases at a high rate: in the 1950s the Court had a Segal-Cover score of +.499, while the current Court has had a score of –.349 since the 2000 term. Although we do not have cases from the 2005 term in our data set, because the Court currently has a score of –.5, our models predict that a corporation has a 54% probability of winning the case—quite a bit higher than the mean rate, which is just 37% over the course of the sixty-four Supreme Court terms we examined.

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