In this paper we investigate whether gender is associated with the content of judicial opinions in the U.S. courts of appeals. Using a topic model analysis, we find that gender is a significant predictor of the content of judicial opinions. Two causal pathways could explain this result: (1) men and women judges write differently about the same cases; (2) men and women judges write about different cases, either due to assignment or selection effects. To untangle these two pathways, we carry out three additional analyses. First, we examine whether the United States as a party is associated with judge gender. We next examine whether case codes are associated with judge gender. Finally, we examine the relationship between topic prevalence and gender, controlling for case codes. Our findings lend greater support to the second pathway than the first. This result raises the prospect that prior work on gender-based differences in judicial behavior may be confounded by assignment or selection effects. Our results also raise normative concerns about gender disparities in voice and influence in the U.S. courts.