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All That Glitters Isn’t Gold
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The increasing use of DNA evidence has revolutionized criminal investigations. Over the past several years, DNA forensics—once thought to be a less reliable identifier than other forensic techniques, such as latent fingerprinting—have now become the evidentiary gold standard in criminal prosecutions. At the same time, non-DNA-based forensic techniques that have incarcerated thousands are coming under fire.

The policy implications of this shifting dynamic—what Michael Lynch and colleagues call an “inversion of credibility”1—can be most clearly seen in the National Research Council’s 2009 report, Strengthening Forensic Science in the United States: A Path Forward. Conducted at Congress’s request by a highly esteemed committee, this report—over three hundred pages—assesses the current state of forensic science.

The committee found remarkable shortcomings in what they call the forensic science knowledge base, noting that the scientific theories and methods used to substantiate many forensic claims frequently cannot withstand close scrutiny. They found an alarmingly “wide variability in capacity, oversight, staffing, certification, and accreditation.”2 For example, lack of transparency, susceptibility to bias, and questionable methodologies for friction ridge analyses (analyses of the prints left by fingers, palms, or soles) make it difficult for two analysts to come to the same conclusion.3 The report’s sobering message is that many forensic applications simply lack scientific rigor despite their routine use in legal proceedings.

Although the committee acknowledges that DNA forensics are not always perfect, the report and its recommendations are framed by an implied yet powerful claim: non-DNA forensic techniques should live up to the gold standard created by DNA typing. But this framing has its own serious drawbacks that obscure much deeper issues concerning both technical matters related to the scientific validity of extending basic DNA identification techniques to novel applications and the ethical, legal, and social implications of DNA forensics’ expanding uses.

DNA Exceptionalism?

The NRC report is replete with both general and specific declarations that frame the accuracy and reliability of DNA typing as the new standard for forensic investigations. At the broad end of the spectrum, the committee notes “DNA typing is now universally recognized as the standard against which many other forensic individualization techniques are judged.”4 This sentiment also shapes the analysis of specific techniques. For example, they note that “overall, the process for tool-mark and firearms comparison lacks the specificity of the protocols for, say, 13 STR [short tandem repeat] DNA analysis.”5

Few seriously doubt DNA typing’s high reliability in determining whether any two isolated samples match. Yet DNA typing is only one of many ways in which DNA analyses are used in forensic investigations. For over two decades, state and federal governments have been collecting convicted felons’ genetic profiles and depositing them into databases in order to be able to identify repeat offenders who leave biological samples at crime scenes.

DNA databases give rise to techniques beyond mere DNA typing that expand criminal investigations’ scope and impact. This repository’s growth—the federal database itself is almost at eight million profiles6—has given rise to three related techniques: cold hits, partial matches, and familial searches.

“Cold hits” occur when investigators are able to match unknown biological materials left at a crime scene with a known database profile. Partial matches occur when investigators identify a suspect using fewer than thirteen loci—the standard number of chromosome sites where, if identical, a “match” between two profiles can be declared. Familial searches work from the premise that relatives share many identical loci. While the number of shared markers between an unknown suspect and a database hit might not incriminate the person with the known profile, it can and has pointed to a relative, who is then the subject of a criminal investigation.

Many assume that these database-oriented techniques have the same precision as typing two individual samples. But what often gets obscured—as it does in the NRC report—is that these newer uses of DNA forensics share many of the same shortcomings that the NRC identifies with non-DNA forensic techniques.

Partial Matches and the Scientific Knowledge Base

DNA databases gain much of their authority from the oft-repeated claim that the...



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