ISSN : 2093-3843
Pieces of evidence that implicate the motive of the defendant for committing the alleged crime do not in itself prove the guilt of the defendant. Davis & Follette (2002) raised a concern that judges and juries would rely on the representativeness heuristic to evaluate the motive evidence and thus, overestimate its probative value which could lead to an erroneous conviction. The present study was to see whether the probative value of a motive evidence is indeed overestimated and to acquire an understanding of the way in which the probative value of a motive evidence is subjectively evaluated. One hundred thirty-one participants in their 20s read a trial scenario of a murder case in which the defendant was charged for a uxoricide. The participants rated the probability of guilt, rendered the verdict, and evaluated the probative value of the motive evidence (infidelity of the defendant) presented by the prosecutor. The trial scenario was manipulated into three levels of the strength of evidence (weak, medium, high), and each level was divided into two conditions, one with the motive evidence and the other without the motive evidence. The participants in the condition with the motive evidence did not render more verdicts of guilty nor rated the probability of guilt higher than the participants in the condition without the motive evidence. On the other hand, the participants' evaluation of the probative value of the motive evidence showed a pattern predicted by the Coherence-Based Reasoning Model (Simon, 2004; Soriano, 2003). Specifically, the strength of evidence affected the participant's conviction about the defendant's guilt and the conviction, in turn, affected the evaluation of the probative value of the motive evidence. Finally, we discussed the admissibility of the motive evidence.
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