Prior knowledge influences interpretations of eyewitness confidence statements: ‘The witness picked the suspect,they must be 100% sure%

When an eyewitness identifies a suspect from a lineup, it is important to know how certain they are about the decision. Even though eyewitnesses are likely to express certainty with words, past research shows that verbal confidence statements (e.g. ‘I’m pretty sure’) are prone to systematic misinterpretation. Until now, no one has examined how an evaluator's prior knowledge, such as which lineup member is the police suspect, influences their interpretation of eyewitness confidence about a lineup identification. Experiments 1 and 3 show that participants perceived the identical statement of confidence as meaning a higher and lower level of certainty, respectively, when the eyewitness's selection either matched or mismatched the police's suspect. Experiment 2 shows that these effects generally persist when the bias manipulation is manipulated between-subjects. Finally, Experiment 3 finds that clarifying the witness's statement with numeric information (e.g. I’m 80% sure) does not eliminate the influence of biasing information.