The more serious an alleged crime before them is, the more convinced jurors are that a defendant is guilty, a new study from Duke shows.
The study was published Monday in Nature Human Behavior.
According to Duke Today, researchers presented six hundred participants with thirty-three cases, from running an illegal distillery to mass murder, with varying amounts of evidence. They were then asked to rate — one a scale of zero to one hundred — how strong each case was via an online assessment. The more severe the crime, the more confident the mock jurors were.
“We found that dognapping was worth 15 points on the scale regardless of the evidence,” said lead study author John Pearson, an assistant professor in the Department of Biostatistics and Bioinformatics in the Duke School of Medicine. “You can think of that as a bias, people mentally move the slider over a certain amount before they see the evidence.”
Online respondents changed their scores by up to twenty-seven points depending on the type of crime. Researchers also conducted the test with law students, lawyers, judges and prosecutors, but found people who were trained to weigh cases strictly on evidence didn’t exhibit the same attitudes, Pearson said.
Surprisingly, prior convictions only changed juror confidence by about ten points. DNA and non-DNA evidence—like fingerprints—were weighted equally by participants at about thirty points, even though the former is more reliable and both can be fallible.
“Fingerprints aren’t as good as DNA,” said senior study author Pate Skene, professor in the Department of Neurobiology within the Duke University School of Medicine. “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.” (Lawyers also gave fairly equal weight to DNA and non-DNA evidence.)
Next, the researchers plan to use MRI machines to measure the brain activity of participants taking the assessment.
Skene told Duke Today the study shows jurors need less evidence to deem a defendant accused of a serious crime guilty, when really more serious cases should require more evidence.
“When the crime is very serious, people perceive it as a threat to themselves or their community,” Skene 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.”