The more severe a crime, the more evidence you should have to prove someone did it. But a new Duke study, appearing in Nature Human Behavior, has shown that the type of alleged crime can increase jurors’ confidence in guilt.
“If the crime is more serious or more heinous, [mock jurors] are more likely to be convinced by the same amount of evidence,” said lead study author John Pearson, an assistant professor in the Department of Biostatistics and Bioinformatics in the Duke School of Medicine.
The researchers made an online computer assessment for participants to read 33 case descriptions, ranging from running an illegal distillery to mass murder, and rate how strong each case was on a scale of 0 to 100. The stronger they found a case, the more likely the mock jurors were to deem defendants guilty.
“We found that dognapping was worth 15 points on the scale regardless of the evidence,” Pearson said. “You can think of that as a bias, people mentally move the slider over a certain amount before they see the evidence.”
Each case included a description of the crime and varying amounts of evidence. Six hundred participants completed the study online. Then the authors continued it with law students, practicing lawyers and judges and active state prosecutors. For the online participants, the type of crime committed changed their scores by up to 27 points.
“That effect goes away with legal training,” Pearson said. “Lawyers are trained that cases are decided by evidence; they don’t care what the person was accused of.”
The study authors also tested how different types of evidence changed participants’ beliefs. Both DNA and non-DNA physical evidence, like fingerprints or fibers, had the biggest effect, contributing about 30 points. Pearson called this the well-documented “CSI effect.” Though DNA evidence is more reliable than non-DNA evidence, jurors trust both equally.
“There’s a certain irony in the fact that the better a scientific method is, the harder it is to remember that it’s sometimes wrong,” said senior study author Pate Skeene, professor in the Department of Neurobiology within the Duke University School of Medicine.
“Fingerprints aren’t as good as DNA,” Skene said. “But they’re pretty good evidence most of the time, so it’s harder to keep in mind that they sometimes lead to mistakes. As the potential for error goes up, it’s more important to point that out.”
Even practicing attorneys gave DNA and non-DNA physical evidence similar weight, with more than 40 points allotted.
Contrary to conventional legal wisdom, knowledge of prior convictions only changed juror confidence by about 10 points. Jurors can only learn about prior convictions if defendants testify, so defense attorneys have to weigh the benefits of testimony against the disadvantages of that knowledge.
The Duke Institute of Brain Sciences funded the collaborative study, which includes law professors and neurobiologists among its authors. Support also came from the National Institutes of Health (K01-ES-025442) and the National Science Foundation (1655445).
Before Skeene started law school at Duke during a sabbatical, he had wondered if the science of decision-making could shed light on how jurors make complex legal decisions.
“We thought you really could apply those decision-making strategies to the stuff I was learning in law school,” Skeene said. “By happy coincidence, my first-year torts professor talked about law and trial practice in ways that made me think about how you could apply cognitive science to study it, and that’s when we thought this was a viable project.”
Next, the authors will do follow-up studies with MRI machines to measure how participants’ brains activate when completing the judgment tasks. They hope to understand how participants’ emotional and moral reactions influence their judgments, Pearson said. For instance, participants may be considering the risks of letting murderers go free versus convicting innocent people.
“When the crime is very serious, people perceive it as a threat to themselves or their community,” Skeene said. “When you get to very serious crimes, the danger of not solving the crime and putting away the person who did it may drive your mental calculus and begin to shift the balance a little bit toward not wanting to take the chance that this person is guilty.”
While the study shows that mock jurors need less evidence to convict serious crimes, they should actually require more, Skeene said.
“That suggests serious attention to that human predisposition to be more confident in more serious crimes and the legal standard of being more careful for more serious crimes,” Skeene said. “We can think about how we conduct trials, how the judge gives instructions to the jury, how the prosecution presents their case [and so on] to mitigate the potential effects of that bias.”
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