The effects of group categorization on statistical inference processes and the consequent effects on group stereotyping were examined in 3 experiments. In Experiments 1 and 2, male and female Ss made data-based judgments about gender and leadership ability. In Experiment 3, Ss were randomly categorized into groups and then made data-based judgments about the groups' relative intelligence. Results from all 3 studies indicated significant effects of group categorization on Ss' judgments and on their strategies of data integration and logical inference. These results support the hypothesis that group members selectively engage in statistical inference strategies as a means of justifying in-group favoritism. Discussion focuses on the implications for understanding group-serving biases, motivated reasoning, and group stereotyping processes.