The Feature Positive Effect in Legal Decision Making: processing and evaluating present absent forensic evidence

On the 29th of May 2001 Kees B. was found guilty for the rape and murder of Nienke Kleiss, a ten year old Dutch girl. There was virtually no forensic evidence that he had committed this crime, but he had confessed, and so the police was convinced of his guilt. The fact that he retracted his confession soon after his crucial interrogation was not heavily weighted by the police. After four years of imprisonment, Kees B. was released, because Wik H. admitted he was the one who killed Nienke (van Koppen, 2003). Indeed, DNA of Wik H. was found at the crime scene. How come Kees B. was found guilty while, besides a striking lack of evidence, DNA found at the crime scene did not match his DNA? And why does one conclude so easily that someone is guilty once his DNA is found at the crime scene? Put shortly, how come one ignores evidence of absence so easily, whereas one gives so much weight to physically present evidence? The phenomenon described above (i.e., ignoring absent information while putting a lot of weight on present information) has been named the ‘feature positive-effect’ (FPE; Jenkins & Sainsbury, 1969), and it is the topic of this dissertation. The FPE originates from literature on Pavlovian conditioning. It was found that pigeons learn associations more quickly when the presence of a stimulus predicts (e.g., blue light is on) the presence of another stimulus (e.g., food can be obtained) rather than when the absence of a stimulus (e.g., blue light is off) predicts the presence of another stimulus. This difficulty with absent information relative to present information has also been described in other domains (e.g., omission bias). The fact that people seem to treat present and absent information differently is not necessarily a bad thing. However, as the example above illustrates, underweighting absent forensic evidence can have severe consequences. Sometimes, the underweighting of absent evidence is irrational and thus constitutes a bias. Unlike other biases that can influence the process of legal decision making, such as the confirmation bias (Nickerson, 1998), the FPE is relatively unknown and unstudied. However, given its possible severe consequences, studying this effect is important.

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