February 17, 1998
Conviction or Acquittal? O.J. Trial Consultant analyzes DNA evidence
UT Professor Presents Findings at AAAS Conference
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Austin, TX -- Depending on how an attorney describes DNA evidence at a trial, jurors will believe the evidence is either irrefutable or unpersuasive, finds Dr. Jonathan J. Koehler, consultant for the defense in the O.J. Simpson criminal trial, and a professor of behavioral decision making at the University of Texas at Austin.
"It’s well known that DNA match probabilities, like all probabilities, may be presented in various mathematically equivalent ways," says Koehler. "What’s not well known is that people’s perceptions about the strength and persuasiveness of DNA evidence are strongly influenced by the way the numbers are presented at trial." These perceptual effects may be the difference between an acquittal and a guilty verdict in cases that rely heavily on DNA evidence.
Koehler will be presenting his research findings at the American Association for the Advancement of Science (AAAS) annual meeting in Philadelphia on Tuesday, February 17. On a panel titled "The Future of Forensic DNA Testing: A Multidisciplinary Examination," Koehler joins Barry Scheck, Paul Ferrara, William Shields, and William Thompson.
In Koehler’s experiments, DNA experts presented evidence in a mock criminal case one of two mathematically equivalent ways. One group of mock jurors was presented with the DNA statistics in the language of probability (e.g. 0.1% and 0.0000001%) in a way that highlighted a suspect’s chance of matching by coincidence. A second group heard the DNA statistics in the language of frequencies (e.g. 1 in 1,000 and 1 in one billion) in a way that highlighted the chance that others would match by coincidence.
Overall, jurors in the first group (single-probability) were impressed with the DNA evidence: a majority of them concluded that the suspect was at least 99% likely to be the source of the blood. However, many jurors in the second group (multi-frequency) believed that there was less than a 1% chance that the defendant was the source of the blood.
In other words, says Koehler, whereas a majority of jurors were quite certain that the suspect was the source of the DNA evidence in the first instance, very few felt this way about the evidence as it was presented in the second instance. "This disparity of opinion occurred despite the fact that both cases were identical in every way except for a single sentence that described the probabilities associated with the DNA match," he says.
A second experiment considered whether this phenomenon would hold up in a real world context in which people have a great deal of additional information and preconceived ideas. Subjects were asked to assume that a dress worn by Monica Lewinsky—the former White House intern alleged to have had an affair with President Clinton—was stained with semen that matched the DNA of the President. The DNA probabilities were then presented in each of the two ways described above. Consistent with earlier studies, Koehler found that in the single-probability case, 28% of the people were 99% certain that the President was the source of the recovered semen, but in the multi-frequency case, that number dropped to 8%.
The implications of Koehler’s findings are monumental: the fate of countless defendants hangs in the balance. Certainly, prosecuting attorneys will want to present DNA evidence in the single-probability perspective, whereas defense attorneys will favor the multi-frequency perspective. Koehler hopes that this research will assist in the development of standards for presenting scientific and statistical evidence in court, and provide an empirical foundation for admissibility rulings and judicial instructions in cases that include quantitative information.
The research, soon to be submitted for publication, also has implications for many other situations in which probabilities play a role. For example, low probability events such as presidential assassinations, bank collapses, or the breakout of war are unlikely to worry many people until they think about them in the multi-frequency perspective: one in 100 banks will go bankrupt this year (hypothetically). Businesspeople may then be much more likely to consider this risk and to take appropriate action.
The AAAS annual meeting is being held at the Pennsylvania Convention Center – Philadelphia Marriot. Koehler’s panel on DNA Testing will run from 8:30 to 11:30 a.m. on Tuesday, February 17.
Contact: Dr. Jonathan Koehler, call 512/892-8843
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