If It Walks Like a Duck: a Case of Confirmatory Bias

Confirmatory bias is an unavoidable source of error in human judgment, which is rooted in the adaptive design of the brain for recognizing meaningful patterns. In forensic psychology, the complete elimination of confirmatory bias is worth aspiring to, but even its substantial reduction is fraught with challenges. In this brief article, I present a vignette from an actual jury trial to illustrate how a seemingly small instance of confirmatory bias led to a major blunder in expert testimony. Also, I consider how it might have been prevented.