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

By now, everyone knows that forensic DNA analysis represents a stunning theoretical advance for the criminal justice system.1 The value of DNA analysis lies in its theoretical power to exclude large proportions of the population as potential contributors of genetic samples (e.g., blood, semen, hair). Thus, when a suspect’s DNA matches a DNA sample that is recovered from the scene of a violent crime, a prosecutor may suggest that the suspect is a likely source of that sample (and was therefore present at the crime scene) because the suspect is among the few who were not excluded as potential contributors. In most cases, the probative value of a DNA match is conveyed to triers of fact via frequency statistics that describe how common the DNA profiles are in a given population.2 A