NCJ Number
200713
Journal
Judicial Officers' Bulletin Volume: 14 Issue: 9 Dated: October 2002 Pages: 66-69
Date Published
October 2002
Length
4 pages
Annotation
This article examines key issues that relate to the presentation and use of DNA evidence in New South Wales courts (Australia), with attention to how easily improper inferences of guilt can be made and how the manner in which DNA evidence is presented in court can unduly influence its interpretation and effect.
Abstract
The "prosecutor's fallacy" or "ultimate issue error" is a distinct issue that has assumed great significance in the presentation of DNA evidence. Confusion often arises when attempting to answer two fundamental questions regarding the probability of a match between the DNA found in crime scene evidence and the DNA of the defendant. The two questions are as follows: What is the probability that the accused's DNA profile matches the crime sample profile, given that he/she is innocent? What is the probability that the accused is innocent, given that his/her DNA profile matches the profile from the crime sample? The fallacious reasoning occurs when the answer to the first question is used to answer the second question. In mathematical terms, this is known as the error of transposing the conditional. Because the answer to the first question is usually small, it is especially significant if the fallacy is committed in the context of DNA evidence in which there are often very small probabilities. Given the weight that jurors often give to the testimony of an expert witness, the misrepresentation of such evidence can be particularly damaging to an accused's case. A related logical fallacy occurs when the defense suggests that identification evidence that involves a characteristic that is shared by a large number of people is of negligible value because "a lot of people could have done it." For example, although a match probability of one in 100,000 in a population of 50 million means that 500 people share a particular characteristic, other evidence that is unique to the circumstances of the case narrows the number of possibilities considerably. The cogency of DNA statistical evidence in criminal trials requires that such evidence be explained accurately and fairly to the jury. Judicial directions that acknowledge there are permissible, equally valid, methods of presenting the same statistical information can restore balance to a jury's assessment. 32 notes