Tanford, S., & Penrod, S. (1984). Social inference processes in juror judgments of multiple-offense trials. Journal of Personality and Social Psychology, 47, 749-765.
According to law, a defendant may be tried for multiple offenses in a single trial, although the courts intuitively recognize that such “joinder” may be prejudicial. The present study investigated the effects of joinder on jurors’ decisions, emphasizing the social psychological mechanisms underlying judgmental biases. 732 representative jurors (aged 18–82 yrs) judged a videotaped trial of a single offense or a joined trial of 3 offenses that varied as a function of (a) charge similarity, (b) evidence similarity, and (c) judges’ instructions designed to reduce judgment biases. A defendant was more likely to be convicted on a particular charge in a joined trial than on the same charge tried alone, and judges’ instructions were ineffective. Joinder produced confusion of evidence and negative inferences about the defendant, but did not significantly influence perceptions of evidence strength. Defendant and evidence ratings were strongly related to verdicts, whereas confusion was not. Results suggest that the effects of joinder are mediated through inferences about the defendent’s criminality.
Conviction rate on focal charge twice as high in joined trial compared with separate trial. More intrusion errors related to testimony recall in joined trial and when charges were similar. Judicial instructions intended to prevent bias ineffective.