Wells Effect

The Wells effect is the finding that a person's stated probability that a defendant is guilty does not translate proportionally into their willingness to actually return a guilty verdict, revealing a disconnect between subjective probability judgments and real decision making in a trial setting. The psychologist Gary L. Wells established the effect through experiments using a hypothetical bus company liability case, in which mock jurors who all reported roughly an 80 percent probability that the company was at fault nonetheless returned guilty or liable verdicts at rates that varied dramatically, from about 8.2 percent to 67.1 percent, depending on what kind of evidence had produced that probability estimate. Wells found this gap was largest when the 80 percent figure came from bare statistical evidence unconnected to the specific facts of the case, compared to evidence that spoke directly to what actually happened, indicating that jurors' verdicts are shaped by more than a rational calculation from a probability estimate alone.