DNA Mixture Analysis – Is It Subjective or Objective?

DNA is often regarded as the gold standard of forensic science because of its purported objectivity, which makes it immune to subjectivity and bias. The National Academy of Sciences 2009 Report, Strengthening Forensic Science in the United States: A Path Forward even concluded that “[w]ith the exception of nuclear DNA analysis, . . . no forensic method has been rigorously shown to have the capacity to consistently, and with a high degree of certainty, demonstrate a connection between evidence and a specific individual or source.” (See p. 7.)

If it were truly objective, contextual information would not affect DNA analysis. In reality, however, when human examiners analyze complex DNA mixtures, the interpretations may be based on subjective judgments and may be influenced by contextual information to fit existing beliefs and expectations.

A recent study by Itiel E. Dror and Greg Hampikian, published in the journal Science and Justice, examined the effect of contextual information on DNA mixture interpretation. The study involved a DNA mixture analysis from a gang rape case in Georgia, where the case analyst had received contextual case information. The researchers then presented the same DNA evidence, without the contextual information, to seventeen independent DNA expert analysts.

In the actual case, one assailant agreed to testify against the other suspects in return for a lighter sentence. According to the Georgia rules of evidence, however, the assailant’s testimony was admissible only if the prosecution offered corroborating evidence. Only the DNA evidence provided the needed corroborating evidence. In the case, the analysts determined that the suspects—including Suspect 3, the primary suspect—could not be excluded from being contributors to the mixture. The analysts knew that without this determination, the informant’s testimony would have been disallowed.

When the researchers presented the original DNA evidence to seventeen independent analysts without the contextual information, with respect to Suspect 3, the independent analysts reached varied results: one concluded that Suspect 3 could not be excluded; four found the analysis inconclusive; and twelve excluded Suspect 3.

This result indicates that subjectivity may be present in purportedly objective DNA mixture analysis. Further, the varied conclusions by the analysts indicate that consistent and reproducible analysis of complex DNA mixtures may not always be possible.

Other DNA interpretation studies have been conducted of several laboratories by Dr. John M. Butler of the NIST laboratories. His studies raise similar issues, including questions about the differing statistical results produced by labs analyzing the same DNA mixture.

The Scientific Working Group on DNA Analysis Methods (known as SWGDAM) acknowledged in their publication (Interpretation Guidelines for Autosomal STR Typing by Forensic DNA Testing Laboratories, 2010) that “[d]ue to the multiplicity of forensic sample types and the potential complexity of DNA typing results, it is impractical and infeasible to cover every aspect of DNA interpretation by a preset rule,” thereby keeping the door open for subjective interpretation by the DNA analyst. This door is also open for knowledgeable experts to challenge interpretation of that data, particularly when it comes to the most difficult DNA samples to interpret, Low Copy Number (LCN) mixtures.

Anand Patel, UNC Law Student and NC State University Ph.D. Candidate contributed to this post.

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