Partisan Voting on the California Supreme Court

Abstract

When did ideology become the major fault line of the California Supreme Court? To answer this question, we use a two-parameter item response theory (IRT) model to identify voting patterns in non-unanimous decisions by California Supreme Court justices from 1910 to 2011. The model shows that voting on the court became polarized on recognizably partisan lines beginning in the mid-1900s. Justices usually did not vote in a pattern that matched their political reputations and party affiliation during the first half of the century. This began to change in the 1950s. After 1959 the dominant voting pattern is partisan and closely aligns with each justice’s political reputation. Our findings after 1959 largely confirm the conventional wisdom that voting on the modern court is on political lines. But our findings call into question the usual characterization of the Lucas court (1987–1996) as a moderately conservative court. Our model shows that the conservatives dominated the Lucas court to the same degree the liberals dominated the Traynor court (1964–1970).

More broadly, this Article confirms that an important development occurred in American law at the turn of the half-century. A previous study used the same model to identify voting patterns on the New York Court of Appeals from 1900 to 1941 and to investigate whether those voting patterns were best explained by the justices’ political reputations. That study found consistently patterned voting for most of the 40 years. But the dominant dimension of disagreement on the court for much of the period was not political in the usual sense of that term. Our finding that the dominant voting pattern on the California Supreme Court was non-political in the first half of the 1900s parallels the New York study’s findings for the period before 1941. Carrying the voting pattern analysis forward in time, this Article finds that in the mid-1900s the dominant voting pattern became aligned with the justices’ political reputations due to a change in the voting pattern in criminal law and tort cases that dominated the court’s docket. Together, these two studies provide empirical evidence that judicial decision-making changed in the United States in the mid-1900s as judges divided into ideological camps on a broad swath of issues.