Andrew H. Stevenson
The Limits of DNA Evidence
DNA evidence is a powerful forensic tool and can play a critical role in a criminal case - either by excluding a person or by providing strong evidence of inclusion. Yet, few truly understand the limits of DNA evidence in a criminal case. We have all had the experience of hearing a news report that a suspect's DNA was found or is a "match." Most immediately jump to the conclusion that the individual "must have done it." Yet, months down the road we learn that no charges are being brought or the person was found not guilty. How is this possible?
The key to understanding the limits of DNA evidence is understanding what DNA evidence can and cannot tell us. The single answer that DNA was designed to answer is: Who did NOT contribute the evidence sample.
DNA analysis consists of comparing the DNA profile found in a piece of evidence (blood, semen, skin, hair, etc) to the DNA profile of a suspect. What DNA evidence is most effective at doing is telling us when a person did NOT contribute the evidentiary sample: it is not the suspect's blood, semen, skin or hair). If the DNA profile (alleles and loci) of the sample evidence do not match the profile of the suspect exactly, the suspect is excluded as a possible contributor. This I call the "prime directive" of DNA evidence.
DNA analysis can in some cases provide strong evidence that a person did contribute the evidentiary sample, but such is never absolute. Rather, DNA analysis is limited to providing odds (often rather astonishingly high ones - 1 in a quintillion). But inclusion (as opposed to exclusion) does have its limits, particularly in cases of low copy number samples.
DNA analysis cannot answer equally relevant questions concerning the relationship between evidentiary and suspect DNA profiles. First, it cannot tell us how the suspect's DNA came to be at a particular location? For example: the story of Mr Lukis Anderson. Second, DNA also has limits when it comes to when. As evidence samples can last a long time (sometimes years), the presence of DNA evidence does not tell us when the suspect contributed the sample. Third, DNA analysis cannot tell us why? The presence of DNA alone, is rarely an explanation of why it is present, and why a person does or does not do something is an important question in determining guilt or innocence. Was the DNA on the gun because the suspect was holding it or because he was taking it from a would-be assailant. Finally, the reliability of DNA analysis for inclusion purposes is also highly questionable in cases of low copy number DNA or very low amounts of evidentiary DNA.
The next time you hear a report on the news that a suspect's DNA was match, remember that this fact is only the beginning of the questions that need to be answered.