Legal standards and the significance of DNA evidence

Most human biologists are aware of controversies regarding the use of DNA profiles in the courtroom. Much attention has been given to estimating the probability of obtaining matches between DNA samples from an innocent suspect and those from a crime scene, but considerably less attention has been given to the critical issue of determining the probability of guilt given a match. Using Bayes' rule and simple algebra, we develop a measure of the strength of DNA evidence that indicates the amount of incriminating evidence needed in combination with DNA match evidence to meet a given conviction standard. Based on current standards and practices, we use this measure to demonstrate that (1) the amount of non-DNA evidence needed to convict, given a DNA match, generally is quite small, even if errors can occur in the processing of DNA evidence; (2) DNA match evidence alone is insufficient to convict, even for the lowest recognized conviction standards; (3) failure to match DNA evidence samples should be exculpatory unless laboratory proficiency is poor; and (4) if errors in handling evidentiary samples occur (even rarely) that tend to produce a false DNA match, then the legal significance of DNA evidence is remarkably insensitive to estimates of chance match probability.