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Two studies purport to show that jurors in criminal trials give too little weight to DNA evidence, but that their skepticism can be overcome by detailed mathematical explanations. The studies offer a possible blueprint for effectively presenting and arguing DNA evidence. The results run counter to a belief among many litigators that too much math and science confuses or alienates jurors. “Jurors do not appear to be overwhelmed by DNA evidence,” said Case Western Reserve University School of Law Professor Dale Nance, who carried out the studies with Scott Morris, a psychology professor at the Illinois Institute of Technology. Rather than exaggerating the value of DNA evidence in comparison with nonscientific evidence, the studies found that jurors are overly cautious about its value because they are aware of their lack of scientific knowledge, of the possibilities of laboratory error and of police planting incriminating evidence. “So they react conservatively by discounting its value,” Nance said. The research involved more than 2,000 people called for jury duty in the criminal courts of Kane County, Ill., between 2000 and 2002. The first report growing out of the research was published last fall in Jurimetrics Journal of Law, Science, and Technology. The other hasn’t been published yet. Each potential juror–they never met in a group as a jury would–was presented a hypothetical case for which the co-authors had predetermined the probability of guilt that a juror ought to assign. Jurors’ actual assessments were then compared with these predetermined probabilities. The authors found that jurors systematically undervalued the DNA evidence. But when an expert witness illustrated “Bayes’ Rule”-a mathematical formula that explains how one should change existing beliefs in light of new evidence-the DNA evidence got wider acceptance and jurors’ verdicts were in line with the authors’ assumptions. An example: After hearing all but the DNA evidence, a juror believed it was a 50-50 probability that a defendant was guilty. If a DNA expert then testified that it was 25 times more likely that the defendant, rather than a randomly selected person, was the source of the DNA, then Bayes’ Rule, illustrated by a chart, was shown to the jury. It showed the juror that his 50% prior belief multiplied by 25 (the additional DNA evidence) came to a 96% probability of guilt. Whether criminal courts will find this number-crunching more probative than prejudicial is doubtful, some criminal lawyers assert. Some practitioners raised objections to the technique the studies endorse. Susan Gaertner, district attorney for Ramsey County, Minn., and a board member of the National District Attorneys Association, believes that the study’s methods “stand principle on its head.” Even though she said that Minnesota courts have been extreme in their resistance to quantifying the weight of DNA evidence, she thinks that the study’s methods are “dangerous to justice.” “Are jurors going to demand 95% certainty and be reluctant to convict if its only 94%? . . . I object to it being a mathematical formula,” said Gaertner, a former co-chairwoman of the DA association’s DNA subcommittee. Peter Neufeld, co-founder and co-director of the Innocence Project at Yeshiva University’s Benjamin Cardozo School of Law, thinks the study’s methods are “ill-conceived because there are too many variables that can’t be taken into consideration.” He said that every attempt to approach nonscientific evidence mathematically in a criminal trial has failed because it lacks a sufficient awareness of the psychological dynamics inside a courtroom. “Prosecutors have successfully exploited junk science and sloppy scientific conclusions without any meaningful challenge from the defense to the delight of jurors,” Neufeld asserted. “In fact, in a third of our wrongful conviction cases, the misapplication of forensic science by the prosecution was a pivotal cause of the miscarriage of justice-the models simply don’t work in the real world,” Neufeld said. Lab error study The second phase of the authors’ research considered how to provide jurors with estimates of the chances of lab error in the finding of a DNA match. It found that after viewing a Bayes’ Rule chart, jurors were still more willing to convict even though the incorporation of the lab error rate reduced the actual probabilities of guilt. Post’s e-mail address is [email protected].

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