Choice and Background Knowledge: How do Individuals Evaluate Accumulating Evidence in A Murder Scenario?

  •  Elizabeth Mackenzie    
  •  Emily Chalmers    
  •  Colin Wastell    
  •  Piers Duncan    
  •  Matthew Roberts    


Can the simple act of selecting a possible suspect of a crime bias the evaluation of the evidence? Does the typicality of the crime impact the assessment of guilt of a suspect? In two experiments, we examine these two questions and find some remarkable results with implications for law enforcement and jury deliberation. Experiment 1 data show that by allowing participants to choose a most-likely-perpetrator, guilt ratings were substantially higher compared to participants who were not allowed to make a choice. This difference persisted after reading a further body of incriminating evidence. In experiment 2 participants were provided with general and specific background information relevant to a suspect, in other words how common was the crime-suspect scenario. When provided with high plausibility compared to low plausibility information, participants gave higher guilt ratings that persisted after further evidence. The results are interpreted in terms of argument theory which provides a parsimonious explanation of the data. These results have implications for the conduct of investigations, for example: putting in place procedures that minimize the effects of suspect prioritization and background information. 

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