Dateline: June, 2007, Issue 4
A key feature of the American legal system is that the litigants control the production and presentation of evidence. Two impacts on evidence presented at trial happen as a result of this key feature. First, the evidence heard at trial is a selective and non-representative sampling of facts offered to suit the interests of the litigants.
Second, investigators frequently generate evidence by combing through large bodies of information for relevant facts. Broad investigations may turn up circumstantial evidence that appears to be highly probative of a claim, but which is actually coincidental. Having multiple opportunities to search for evidence increases the chance of finding seemingly supportive evidence by chance alone, even when the claim being forwarded is actually false.
Two recent experiments conducted by Koehler and Thompson (2006) examined whether jurors appreciate that multiple searches may generate seemingly supportive evidence by chance alone, and whether the evidence they hear at trial is a selective and unrepresentative sample of the underlying facts.
In both experiments, jurors attached less weight to a case against a defendant when they knew the evidence resulted from a broad, multiple-opportunity search than when it resulted from a narrow search. Jurors who were ignorant about the breadth of the search viewed the case similarly to jurors who knew the evidence resulted from a narrow search.
These researchers concluded that jurors understand that broad discovery with multiple opportunities to find supportive evidence is likely to find seemingly supportive evidence by chance alone. However, jurors may not consider the possibility that evidence presented at trial was strategically selected from a larger array of facts unless that larger array is brought to their attention and identified for them. Said differently, if failed investigative leads are not discussed, jurors may give more weight to a case than they would otherwise.
Source Koehler, J. J. & Thompson, W. C. (2006). Mock jurors' reactions to selective presentation of evidence from multiple-opportunity searches. Law and Human Behavior, 30, pp. 455-468.