Under federal and state laws, a defendant who has been charged with more than one offense can be tried for all the offenses in a single “joined” trial. It was predicted that the probabillty a defendant would be convicted would increase as a function of the number of joined offenses. Legal theories, research on memory, and social psychological models of information integration and attribution led to three hypotheses as to why this bias might occur: (1) confusion of evidence, (2) accumulation of evidence, and (3) inference of a criminal disposition. Subjects read and judged written trial summaries presented as joined or single trials. In Study 1, joinder resulted in higher rates of conviction and in confusion of evidence. In Study 2, the conviction results were replicated, and subjects judging joined trials also rated the evidence as more incriminating and made negative attributions about the defendant. These ratings were strongly related to judgments of guilt. A sequential judgment process was also found to affect jurors' judgments.