The Psychology of Numbers in the Courtroom: How to Make DNA-Match Statistics Seem Impressive or Insufficient

Abstract

A DNA match statistic of, say, one in one million means that approximately one person out of every one million in a population will match that DNA profile. Now consider a juror who hears that one in every one hundred thousand people in, say, Houston who are not the source will match coincidentally. Half of the subjects (selected at random) received the following target / probability frame (s/p) wording: The probability that Mr. Clinton would match the genetic material if he were not the source is 0.1%. As in the Clinton-Lewinsky study, the information provided was identical for all jurors with the exception of a single sentence that described the DNA match statistic. Nevertheless, if future studies confirm the trends detected here, the legal system must take seriously the idea that the way in which a match statistic is worded by an expert or attorney can affect the way a juror thinks about the value of that evidence. At the one in one million incidence rate, there were small differences between the single target and the multi target on p(source) estimates (81% vs. 76%). Second, the reluctance of some jurors to assign extremely high p(source) and p(guilt) values in mock DNA cases is now a fairly consistent finding in the mock juror literature.

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