Friday, September 23, 2011
In State v. Wright, 253 P.3d 838 (Mont. 2011), the Montana Supreme Court rejected a defendant's claim that allegedly false and misleading expert testimony and closing argument about the number of individuals with a particular DNA profile who lived in the state amounted to a violation of due process of law. However, the opinion does not adequately articulate the nature of the error in the testimony and is ambiguous in its discussion of the appropriate limits on the admission of DNA source-attribution testimony. This essay explains that the expert's testimony is an instance of an "expected value fallacy." This fallacy consists of thinking that the expected number of matching DNA profiles necessarily is the only plausible number of such profiles.