Statistical inference and forensic evidence: evaluating a bullet lead match

This experiment tested the ability of undergraduate mock jurors (N=295) to draw appropriate conclusions from statistical data on the diagnostic value of forensic evidence. Jurors read a summary of a homicide trial in which the key evidence was a bullet lead "match" that was either highly diagnostic, non-diagnostic, or of unknown diagnostic value. There was also a control condition in which the forensic "match" was not presented. The results indicate that jurors as a group used the statistics appropriately to distinguish diagnostic from non-diagnostic forensic evidence, giving considerable weight to the former and little or no weight to the latter. However, this effect was attributable to responses of a subset of jurors who expressed confidence in their ability to use statistical data. Jurors who lacked confidence in their statistical ability failed to distinguish highly diagnostic from non-diagnostic forensic evidence; they gave no weight to the forensic evidence regardless of its diagnostic value. Confident jurors also gave more weight to evidence of unknown diagnostic value. Theoretical and legal implications are discussed.