|When an expert speaks, what does the jury hear?|
Overall, as might be expected, jurors placed more emphasis on the expert testimony and used it in their verdict of guilt than judges did.
However, both judges and jurors were more impressed with expert testimony that had a strong "qualitative" feel to it. For example, if the expert said the hair "matched" the defendant or was "similar in all microscopic characteristics," judges and jurors gave that testimony a lot of weight. They gave less weight to expert testimony stating the defendant's hair was "similar to the hair found at the murder scene, although there are a number of people in the community whose hair would also have the same identifying attributes." The "match" or "similar in all microscopic characteristics" testimony sounds much more confident than the "other people have the same microscopic characteristics" approach.
So, what do you do with this information? I suppose the prosecution might want to talk to their expert about asserting their opinions in qualitative, confident ways (without distorting the truth, of course). The defense will want the expert to then discuss the limits of the forensic testing used in cross examination.
Warning for the defense: This same research study found that having experts talk about the limits of testing did not affect jurors' opinions all that much.
Warning for the prosecution: If the defendant opts for a bench trial, this study found that judges do not let the "match" or "similar in all microscopic characteristics" testimony affect their estimation of guilt as nearly as much as jurors do.
Thanks for reading-- Maximillian Wachtel, Ph.D.
McQuiston-Surrett, D., Saks, M. J. (2009). The testimony of forensic identification science: What expert witnesses say and what factfinders hear. Law and Human Behavior, 33(5), 436-453, doi:10.1007/s10979-008-9169-1