Confirmation Bias in the United States Supreme Court Judicial Database

This paper investigates the possibility of confirmation bias in the United States Supreme Court Judicial Database (USSCJD) issue and judgment codes. We ask whether an opinion issued by a liberal Court is more likely to be assigned a USSCJD issue code that leads to a liberal judgment code, relative to an otherwise similar opinion issued by a conservative Court (and vice versa). Using a sample of cases from the USSCJD that pose comparable issue coding choices, we find that cases are disproportionately assigned issue codes that tend to lead to judgment codes confirmatory of expectations about the ideological character of the judgments typically issued by the deciding Court. We also find considerable evidence that variation in the Court’s decision making as a function of congressional preferences has been “coded out ” of the USSCJD as a result of confirmation bias in the issue codes. Finally, we recode a subset of the USSCJD judgment codes to eliminate confirmation bias. We find that this bias may have led many researchers using the original USSCJD judgment codes to reject the hypothesis of congressional constraint on the Court, despite compelling evidence for the existence of such constraint using the recoded judgment codes.