Forensic Confirmation Bias

Forensic confirmation bias is a finding in forensic science research that a forensic examiner's judgment about an ambiguous piece of physical evidence, such as a fingerprint, bite mark, or handwriting sample, can be measurably shifted by contextual information the examiner is exposed to about the case, such as learning that a suspect has already confessed or that other evidence points toward a particular person, causing the examiner's interpretation of the ambiguous evidence to move toward confirming that prior expectation even when the examiner has no intention of being biased and genuinely believes their own analysis to be objective. The psychologist Itiel Dror conducted influential research demonstrating the effect using experienced fingerprint examiners, finding that the very same examiner could reach different conclusions about an identical pair of prints depending on what contextual information about the case had been provided alongside the prints, despite the physical evidence itself never actually changing. The phenomenon has since been discussed extensively in reviews addressing sources of error across the criminal justice system, including a widely cited 2013 review by the psychologists Saul Kassin, Itiel Dror, and Jeff Kukucka surveying confirmation bias at multiple stages of a criminal investigation, and it has contributed to reforms in some forensic laboratories aimed at shielding examiners from case information not strictly necessary for their specific analysis.