The testimony of forensic identification science: what expert witnesses say and what factfinders hear.

This research examined how variations in the presentation of forensic science information affect factfinders' judgments in a trial. Participants read a summary of a murder case, the critical testimony being the results of a microscopic hair comparison given by a forensic expert. Across two experiments we manipulated how the expert expressed his results, whether he gave an explicit conclusion concerning identity of the hair, and whether the limitations of forensic science were expressed during trial. Qualitative testimony was more damaging to the defense than quantitative testimony, conclusion testimony increased the defendant's culpability ratings when findings were presented quantitatively, and expressing limitations of forensic science had no appreciable effect. Results are discussed in terms of factfinders' interpretation of forensic identification evidence.