Thu, 20 Aug 2009

DNA, a forensic silver bullet?

DNA evidence is perceived by many to be virtually infallible, a truth machine in criminal investigations. This is not the case. Many of the fallacies about DNA's forensic powers are detailed in The Potential for Error in Forensic DNA Testing (and How That Complicates the Use of DNA Databases for Criminal Identification) (pdf), a very clear and easy to read paper published last year by Professor William C. Thompson, Department of Criminology, Law & Society, University of California, Irvine:

The infallibility of DNA tests has, for most purposes, become an accepted fact—one of the shared assumptions underlying the policy debate.

In this article, I will argue that this shared assumption is wrong. Although generally quite reliable (particularly in comparison with other forms of evidence often used in criminal trials), DNA tests are not now and have never been infallible. Errors in DNA testing occur regularly. DNA evidence has caused false incriminations and false convictions, and will continue to do so. Although DNA tests incriminate the correct person in the great majority of cases, the risk of false incrimination is high enough to deserve serious consideration in debates about expansion of DNA databases. The risk of false incrimination is borne primarily by individuals whose profiles are included in government databases (and perhaps by their relatives). Because there are racial, ethnic and class disparities in the composition of databases, the risk of false incrimination will fall disproportionately on members of the included groups.

This article will discuss major ways in which false incriminations can occur in forensic DNA testing, including coincidental DNA profile matches between different people, inadvertent or accidental transfer of cellular material or DNA from one item to another, errors in identification or labeling of samples, misinterpretation of test results, and intentional planting of biological evidence. It will also discuss ways in which the secrecy that currently surrounds the content and operation of government databases makes these issues difficult to study and assess. It will conclude by calling for greater openness and transparency of governmental operations in this domain and a public program of research that will allow the risks discussed here to be better understood.


Do innocent people really have nothing to fear from inclusion in government DNA databases? It should now be clear to readers that this claim is overstated. If your profile is in a DNA database you face higher risk than other citizens of being falsely linked to a crime. You are at higher risk of false incriminations by coincidental DNA matches, by laboratory error, and by intentional planting of DNA. There can be no doubt that database inclusion increases these risks, the only real question is how much. In order to assess these risks, and weigh them against the benefits of database expansion, we need more information.

It is well worth taking the time to read this article in full. The risk of false incrimination from DNA evidence is only one of the reasons, especially for innocent or those arrested for some trivial matter, not to want to be on a DNA database such as the NDNAD. Other objections include the stigma of being on a criminal intelligence database (an honorary criminal), risks to privacy, database function creep and other misuse by the database state, and concerns about the efficacy of retaining DNA of individuals (only retention of DNA from crime scenes has been shown to drive detections).

The risk of intentional planting of DNA by criminals has been in the news as The New York Times reported earlier this week that scientists in Israel "showed that if they had access to a DNA profile in a database, they could construct a sample of DNA to match that profile without obtaining any tissue from that person." Simpler scenarios include planting biological evidence taken from someone else. Yet another technique developed at the University of Western Australia is to create a spray made from someone else DNA which, under certain condition, can mask the profile of the actual DNA found in a bloodstain or other samples from crime scenes. William C. Thompson reiterates the risks that we may face if or when criminals start using these techniques:

If someone plants your DNA at a crime scene, it might throw police off the trail of the true perpetrator, but it is unlikely to incriminate you unless your profile is in the database. The authorities are likely to search the profile of the crime scene sample against a database, but if your profile is not in the database, they will find no match and will be left with just another unknown sample. Suppose, however, that you are unlucky enough to have your profile in the database. In that case, the police will likely find it, at which point they will have something far better than an unknown sample—they will have a suspect. Given the racial and ethnic disparities that exist in databases, that suspect is disproportionately likely to be a minority group member.


Kary Mullis, who invented PCR [technique used to create highly concentrated solutions of DNA fragments, such as in the spray scenario], anticipated this potential misuse of the technique. In a conversation I had with him in 1995, Mullis jokingly discussed creating a company called “DN-Anonymous” that would sell highly amplified solutions of DNA from celebrities, or from large groups of people, that criminals could use to cover their tracks. Although Mullis was not serious about doing it himself, he predicted that someone would do so within the next ten years. As far as I know, Mullis’ prediction has yet to come true, but it may only be a matter of time before materials designed to stymie DNA tests (by planting other people’s DNA at crime scenes) become available for sale on the internet along with kits designed to thwart drug tests.

Another incorrect assumption is to believe that everyone has only one DNA profile. Amazingly, for a small number of persons this is not true: analysis of different samples from the same individual can result in distinct DNA profiles. In some bone marrow transplant treatments, patients' blood contain a mixed DNA profile. "Chidambaram [of the Alaska State Scientific Crime Detection Laboratory in Anchorage] argues that potential marrow donors should be informed of the small risk of their DNA profile turning up in a crime database if the recipient later commits an offence." Another instance where the unique DNA profile assumption doesn't hold is for those affected by the rare chimerism condition, when two fertilised eggs fuse to become one foetus. Chimeras end up with two separate strands of DNA. Depending on what DNA sample is taken, one of two distinct DNA profiles may be obtained.

DNA evidence has proved useful in criminal investigation but it is no silver bullet. Mistakes are made, DNA evidence can be planted at crime scenes, it can even be fabricated and some persons have more than one DNA profile. Limitations in the evidential powers of DNA need to be understood by judges and juries... and the Home Office.

For a better understanding of the Home Office plans, check out some of the several public responses to Home Office consultation collated by GeneWatch UK. Most of the responses and advice currently available are deeply critical of the science and/or the legality (pdf) of the Home Office's proposals for 'Keeping the right people on the DNA database'.

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