Koehler, J. J. (2001). The Psychology of Numbers in the Courtroom: How to Make DNA Match Statistics Seem Impressive or Insufficient. Southern California Law Review, 74, 1275-1306.

ABSTRACT

This article describes a series of studies that I conducted with hundreds of jury-eligible subjects in Austin, Texas. The studies examine factors related to the presentation of DNA statistics that affect whether a juror will be impressed with and persuaded by the DNA evidence. The data are described in the context of a novel psychological theory for predicting when legal decision makers will and will not be impressed and persuaded by statistical evidence. Section I lays out the theory of interest, "exemplar cueing theory." Sections II, III and IV are three controlled behavioral experiments that develop and test key components of the theory. These experiments show how subtly different presentations of the identical DNA match statistics in a murder case affect (a) estimates of the chance that the suspect is the source of the matching DNA evidence, (b) estimates of the chance that the suspect is guilty, and (c) verdicts. One conclusion that emerges is that DNA match statistics that target the individual suspect and that are described as probabilities (e.g., "The probability that the suspect would match the blood drops if he were not their source is 0.1%") are more persuasive than statistical presentations that target a broad suspect population (e.g., all people in a large city) and that are framed as frequencies (e.g., "One in 1,000 people in Houston would also match the blood drops"). Section VI contains a fourth controlled experiment that identifies some limiting conditions on target and frame effects that may have practical importance in the courtroom. This section also includes discussion of why mock jurors may not appear to be persuaded by statistics as small as 1 in 1 billion. Section VI is a brief summary of the experimental findings and a discussion of experimental limitations. Section VII is a broader discussion of the psychology of DNA statistics. After examining the difference between legitimate and illegitimate statistical forms of DNA statistics, Section VII discusses how prosecutors and defense attorneys can use exemplar cueing to their advantage in the courtroom. Section VIII is a brief closing comment.