The leading Federal case on joinder, United States v. Foutz (1976), recognized three possible sources of prejudice when a defendant is tried for two or more offenses that occurred at different times and places: jurors may confuse evidence presented in proof of different charges, accumulate or combine evidence across different charges, and infer that the defendant has a criminal disposition because he or she is charged with multiple crimes. To evaluate these hypotheses, qualified juror subjects viewed realistic videtaped trials involving a single offense or a joinder of three charges that varied according to charge similarity, evidence similarity, and judges' instructions. Subjects gave an individual verdict preference; deliberated in groups of six and reached a group decision; and responded to a questionnaire designed to assess memory, accumulation of evidence, and criminal inferences. A supplementary study replicated the experimental conditions, using undergraduate students who did not deliberate. The results show that joinder led to a certain amount of confusion of evidence; an accumulation of evidence, particularly against the defendant; and negative inferences about the defendant. In the major study using jurors, convictions increased regardless of charge and evidence similarity, and a very strong set of judge's instructions had no effect on the verdict. While the best guideline is to avoid joining charges at all, a second would be to adopt stringent standards for application of rules such as Rule 404(b). Tables and approximately 90 references are supplied.