Confirmation Bias in Criminal Cases

Abstract: Confirmation bias is a tendency to selectively search for and emphasize information that is consistent with a preferred hypothesis, whereas opposing information is ignored or downgraded. This thesis examines the role of confirmation bias in criminal cases, primarily focusing on the Swedish legal setting. It also examines possible debiasing techniques.Experimental studies with Swedish police officers, prosecutors and judges (Study I-III) and an archive study of appeals and petitions for new trials (Study IV) were conducted. The results suggest that confirmation bias is at play to varying degrees at different stages of the criminal procedure. Also, the explanations and possible ways to prevent the bias seem to vary for these different stages. In Study I police officers’ more guilt presumptive questions to apprehended than non-apprehended suspects indicate a confirmation bias. This seems primarily driven by cognitive factors and reducing cognitive load is therefore a possible debiasing technique. In Study II prosecutors did not display confirmation bias before but only after the decision to press charges, as they then were less likely to consider additional investigation necessary and suggested more guilt confirming investigation. The driving forces need further examination. Study III suggests that pretrial detentions influence judges’ perception of the evidence strength, making them more likely to convict, in cases where they themselves detained. This is indicative of a confirmation bias with social explanations, which, possibly, can be mitigated by changing decision maker between detention and main hearing. The confirmatory reasoning in Study I-III can be considered rational or irrational, following different types of rationality, like probabilistic or judicial rationality. In Study IV, statistical estimates based on empirical data from the Apellate Courts and the Supreme Court indicate that far from all wrongfully convicted who appeal or petition for a new trial are acquitted. A robustness analysis confirmed that these overall conclusions hold over a wide variety of assumptions regarding unknown parameters.                         Also, the usage of empirical methods to study law and legal phenomena is discussed. The concept of Evidence-Based Law (EBL) is used to exemplify how empirical legal research may benefit both legal scholarship and law in a wider sense.