- DNA testing has been widely recognized as extremely reliable, and the results of a properly conducted DNA comparison are admissible.
- The results of a DNA comparison may be expressed in terms of probability (e.g., “the odds are 1 in 10,000,000 that the DNA collected from the scene would match to someone selected at random, other than the defendant”), but this may not be conflated to say “therefore the odds are 10,000,000 to 1 that the defendant is guilty.”
- If the state complies with the statutory notice requirements then it may admit the forensic analysis report without calling a live witness, but this is not usually recommended since the jury needs to hear an explanation of how the process works.
Admissibility and Reliability
In State v. Pennington, 327 N.C. 89 (1990), the North Carolina Supreme Court considered for the first time whether evidence of deoxyribonucleic acid (“DNA”) testing was properly admitted as a new scientific method. In finding that the trial court did properly admit the evidence, the court ruled there was uncontroverted expert testimony showing that DNA testing used established techniques considered reliable by the scientific community, and it was therefore admissible.
Numerous subsequent decisions have confirmed the admissibility of this type of evidence. See, e.g., State v. McKenzie, 122 N.C. App. 37 (1996) (quoting Pennington, “[o]ur Supreme Court has held that DNA profile testing is ‘generally admissible.’”); State v. Hairston, 123 N.C. App. 753 (1996) (witness was properly qualified as expert in DNA analysis); see also State v. Underwood, 134 N.C. App. 533 (1999) (mitochondrial DNA [“mtDNA”] testing is also scientifically reliable); State v. Bruno, 108 N.C. App. 401 (1993) (when there is conflicting testimony from more than one DNA expert, jury determines credibility of their conclusions).
More recent cases decided after the 2011 Daubert amendments to Rule 702 have continued to admit expert testimony about DNA analysis. See, e.g., State v. Messer, 255 N.C. App. 812 (2017) (“Agent Michelle Hannon, a DNA analyst at the State Crime Laboratory […] tested the evidence against the DNA from Defendant's cheek swab. In her expert opinion, the blood on Defendant's shirt matched the DNA profile of Billy Strickland.”); State v. West, 255 N.C. App. 162 (2017) (state’s evidence included “expert testimony that Defendant's DNA matched a genital swab taken from D.S. shortly after the alleged assault”); State v. Flood, 250 N.C. App. 509 (2016) (unpublished) (applying amended Rule 702, and concluding that “the trial court did not commit plain error by admitting the expert testimony concerning the DNA profiling from the DNA sample discovered on the concrete slab of Defendant's apartment”).
For a more detailed discussion about the science behind DNA and best practices for collecting samples and performing testing using single source samples, simple mixture samples, and complex mixture samples, see the President’s Council of Advisors on Science and Technology (“PCAST”) 2016 report, Section 5.1, pages 69-83. In short, “DNA analysis of single-source and simple mixture samples includes excellent examples of objective methods whose foundational validity has been properly established.” Id.
Expressing Results and Relative Degree of Certainty
After a qualified analyst extracts and compares the DNA profile from a sample (such as a swab collected at the crime scene or taken from another piece of evidence) to the DNA profile of the defendant and identifies those two profiles as being a “match,” the witness will likely quantify the significance of that match by referring to “random match probability.” In other words, the witness will explain that the likelihood that a member of the general population selected at random would share the same DNA profile found in the test results (that is, be a “match”) is only 1 in X,000,000. See State v. Watts, 172 N.C. App. 58 (2005) vacated in part on other grounds, 361 N.C. 161 (2006) (court ruled, relying on State v. Futrell, 112 N.C. App. 651 (1993), and other cases, that the state’s DNA expert was properly permitted to testify about “population statistics” and the relative likelihood that the DNA would match any individual other than the defendant, and to conclude that “it was scientifically unlikely that the semen found on the minor's underwear originated from anyone other than Defendant”); see also State v. Hill, 116 N.C. App. 573 (1994) (Witness characterized the four matches found in this case as an extremely rare result; court rejected defendant’s argument that witness in effect improperly stated his opinion that the defendant was the person who committed the rape. Witness also testified that the possibility of selecting another unrelated individual having the same profile as the defendant was approximately 1 in 2.6 million for North Carolina’s white population. The court rejected the defendant’s argument that the database was too small to permit the use of statistical analysis concerning the probability estimate.) For more information and discussion on this topic, see “DNA Test Results: Probability vs. Fallacy,” Jonathan Holbrook, North Carolina Criminal Law Blog, May 8, 2018.
Before presenting the results of DNA testing and analysis in court, there are two important issues that the proponent of the testimony needs to understand regarding certainty and probability.
First, if the sample being analyzed was degraded, poorly collected/preserved, or contained a mixture from more than one source of DNA, then the expert’s opinion may be less conclusive. But as long as that opinion is still relevant and helpful to the finder of fact in resolving an issue, it should be deemed admissible. See State v. Best, 342 N.C. 502 (1996) (agent testified that the DNA sample was degraded and difficult to separate from victim's DNA and the tests were therefore inconclusive, but they did “not count [the defendant] out” and would eliminate approximately ninety-four of one hundred people from the black population; court ruled that the testimony was relevant and admissible).
Second, the “random match probability” must not be interpreted or expressed as the “odds of guilt” of the defendant. This mistake in logic is often referred to as the “prosecutor’s fallacy,” and testimony or argument based on this fallacy is improper. In other words, if the witness opines that the odds of a random probability match are 1 in 1,000,000, this may not be expressed to the jury as “this means there is a 99.999% chance the defendant did it, and only a .001% chance that he didn’t.” See McDaniel v. Brown, 558 U.S. 120 (2010); State v. Ragland, 226 N.C. App. 547 (2013).
Explaining the fallacy
To better understand this logical fallacy, prosecutors may want to think about why it would be improper (and statistically inaccurate) for the defense to try to argue the reverse position. For example, the defense might say that if the odds of a match are 1 in 10,000,000, and the community has a total of 100,000,000 people, it means that one would expect at least 10 different people to “match” the sample from the crime scene. That means the defendant is no more likely to be guilty than the other 9 matching people – so there is actually a 90% chance (9 out of 10!) that he’s not the perpetrator….
Obviously that isn’t correct, because it fails to factor in all the other evidence in the case (motive, opportunity, eyewitnesses, statements, fingerprints, etc.) that also points to this particular defendant.
To put it another way, the fallacy arises from a failure to recognize that there are two different questions at issue: (1) what is the probability of a match, assuming the defendant is innocent; and (2) what is the probability the defendant is innocent, given that there is a match? The “prosecutor’s fallacy” uses the answer to the first question as an answer to the second.
Forensic Analysis Admissible Without Live Testimony
G.S. 8-58.20 establishes procedures allowing the state to submit a laboratory report of a forensic analysis (including comparison and analysis of the defendant’s DNA) without calling the analyst as a witness. It requires that: (i) the forensic analysis meet certain standards of accreditation and procedure; (ii) the analyst complete an affidavit containing certain averments (for example, that the analyst is qualified to perform the analysis); and (iii) the district attorney provide a copy of the report and affidavit to the defendant’s attorney (or to the defendant if he or she has no attorney) within the time specified in the statute.
If the defendant fails to file a written objection with the court within 15 business days of receipt of the report and affidavit, the state may introduce the report and affidavit without calling the analyst as a witness unless the presiding judge rules otherwise. If the defendant timely objects, the state may not introduce the forensic analysis unless it would otherwise be admissible. These notice and objection procedures apply to proceedings in both district and superior court.
Even though it may be statutorily permissible to have DNA evidence admitted without the live testimony of a witness, this practice should generally be avoided unless it is absolutely necessary. The science behind DNA testing and analysis is complex, and it is almost always better for the state if there is a live witness in court who can help explain it to the jury.
Other Statutory Provisions Concerning DNA Evidence
G.S. 15A-267: Criminal defendant’s access to DNA samples from crime scene
G.S. 15A-268: Preservation of samples of biological materials
G.S. 15A-269: Criminal defendant’s request for postconviction DNA testing