Supreme Judicial Court (February 1, 2010)
Expert testimony that DNA test results could not exclude the defendant as a potential source of DNA found at the crime scene must also include testimony regarding statistical findings explaining the importance of such a result. The absence of this testimony is unduly prejudicial and constitutes reversible error.
The defendant was convicted of home invasion and assault with intent to rape, among other charges. The defendant appealed, arguing the judge committed reversible error by admitting expert testimony that DNA tests could not exclude the defendant as the source of DNA taken from the crime scene without accompanying testimony explaining the statistical import of those results. The SJC granted the defendant's application for further appellate review and agreed with the defendant. The case was remanded for a new trial.
The Commonwealth presented expert DNA "match" testimony which included the statistical importance of those matches. The evidence that was determined a "match" however, was evidence that did not directly implicate the defendant. The defendant did not challenge the "match" testimony. The DNA expert also testified on other tests conducted on samples taken from the defendant's sweatpants, and on a swabbing of blood found on the interior doorknob of the door to the victim's apartment. These tests did not result in "matches" but could not exclude the defendant or victim as a possible contributor to the sample. As to both pieces of evidence, the expert testified that the victim and the defendant were both "included" as "potential contributors" to the mixture, i.e., neither the victim nor the defendant "were excluded as a potential contributor to the DNA mixture." The expert did not testify as to the probability that an individual randomly selected from the population would also "not be excluded" by these tests, nor did she provide any other indication as to the meaning of a "not excluded" result. This was error.
The SJC previously held that DNA test results showing a DNA match would not be admissible unless the jury was also told about the likelihood of that match occurring. "[E]vidence of a match based on correctly used testing systems is of little or no value without reliable evidence indicating the significance of the match, that is, 'evidence of the probability of a random match of [the victim's or] the defendant's DNA in the general population.' " Comm. v. Rosier, 425 Mass. 807, 813 (1997). The same reasoning applies to evidence that a DNA test, although resulting in less than a complete "match," could not exclude a particular individual as a potential contributor. The jury has no way to evaluate the meaning of the result without this statistical evidence. The SJC held that admitting this evidence without accompanying evidence that properly interprets those results creates a greater risk of misleading the jury and unfairly prejudicing the defendant than admission of a "match" without accompanying statistics. The conviction was reversed and a new trial was ordered.