Reasonable Doubt: The drawbacks with DNA dragnets

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      First of two columns

      Since the early 1980s, DNA has vastly improved the ability of police to identify and convict perpetrators of serious crimes such as rape and murder. DNA profiling helps identify a specific set of characteristics unique to each person, much like fingerprints, and is therefore a valuable investigative and evidentiary tool. DNA has helped solve cold cases and has exonerated some wrongfully convicted.

      Once a DNA profile is identified, either from the crime scene or a victim’s body, the police have useful evidence to help them match a suspect’s DNA with that DNA profile. The police maintain a DNA database that they use to search for full or partial matches using special techniques. Once they have a possible match, they then have a suspect whose DNA is collected for a more accurate comparison.

      If, however, their search fails to create any leads, they go out to the community and sometimes use what are called DNA dragnets, or DNA sweeps. When using this investigative tool, the police target a certain segment of the community, such as a victim’s known associates, family, current or past romantic partners, coworkers, etcetera, and ask them to voluntarily provide a DNA sample.

      Provide sample—cleared; refuse sample—suspect

      There are significant benefits to using DNA profiling. When engaging a DNA dragnet, the theory is that only guilty parties would refuse to voluntarily provide a sample. Those that provide a sample are cleared, and those who refuse are immediately labelled suspects.

      It is difficult to argue that DNA dragnets are a bad thing if they result in either the identification of suspects or the conviction of the actual culprit. Forensic science such as DNA profiling is undoubtedly a valuable resource, based on the fact it provides theoretical probabilities such as a one-in-one-million, -billion, or -trillion chance that the person whose DNA matches the DNA found at the crime scene is someone else’s. This provides a fair amount of certainty that the person whose DNA matches the DNA profile is the person who committed the crime.

      However, there are potential downsides to using DNA dragnets, which, in fact, rarely result in identifying the actual perpetrator and take an enormous amount of time and money to process. This two-part article will outline both the practical and scientific concerns with this investigative technique.

      Public perceptions of guilt a big factor

      First, on the practical side of things, not everyone who refuses to voluntarily provide a DNA sample is guilty of the crime. Many factors could influence an individual’s decision not to provide a sample, including the alienation of the targeted group, public presumptions of guilt even after a suspect has been cleared, and privacy concerns.

      There is also the chance that the suspect does not want his or her DNA sampled because he or she may have committed a different crime. Being requested to provide a sample is similar to being put in the position of having to prove one’s innocence; each person is not considered innocent until they provide the sample and are cleared of any wrongdoing. These practical concerns will be described below.

      Scientific concerns understated

      Second, on the scientific end of things, the theoretical probabilities of the certainty of a match are overstated when taking into account such factors as identical twins, chimeras, limitations in DNA profiling software requiring subjective input by forensic scientists, the possibility of contamination, and the risk of inaccurate DNA profiles derived from mixed DNA samples. These concerns will be outlined in next week’s discussion.

      Turning again to the practical concerns, alienation of the targeted group may occur as a result of the public’s reaction or when tension exists between the community and the police. An example is the recent DNA dragnet used in Prince George, B.C. The investigation was in connection to the inquiry into missing women along Highway 16, now known as the “Highway of Tears”.

      While implementing a DNA dragnet, the RCMP requested samples from all taxi drivers, the theory being that there was a strong likelihood that some of the missing women would have had last contact with a taxi driver. The public responded by becoming suspicious of all taxi drivers because they had been asked to provide DNA in connection to the crimes.

      Police tactics may seem coercive

      Many of the cabbies had initially wanted to refuse providing samples but quickly gave in once they realized that if they didn’t, they would become a suspect. The community was already at odds with police in the area. Adding to that tension was the fact that taxi drivers felt coerced after initially believing they were invited to speak to police because they may have had useful information, making them less willing to cooperate. The result undermined the investigation. To date, none of the samples have resulted in any leads.

      In another DNA dragnet involving a female who had been raped and murdered, a young man voluntarily provided a DNA sample after being asked to do so because he matched the physical characteristics of race and height of the unknown male suspect’s DNA profile. He was cleared of any wrongdoing months later, but in the meantime he lost his job and coworkers began to view him as a rapist and murderer because of the police scrutiny. What’s more, he had to go to court to force the authorities to return his DNA sample, which they had refused to turn over even after the killer was identified.

      Sample retention, DNA databases a concern

      The potential for DNA samples to be retained and entered into DNA databases is a significant concern. In the United Kingdom, for example, DNA samples are collected and kept from anyone accused of a crime, not only those convicted. Currently in Canada, only those who are convicted of more serious crimes have their DNA entered into the database, and for the time being, samples that do not result in leads are destroyed. But how long will it be before Canada follows suit with the U.K.?

      Furthermore, once someone’s DNA is collected, there is no guarantee that a record is not kept in some form for which there are no standards of control to safeguard privacy rights.

      Another concern is that in the future, insurance companies may be able to access the profiles. They may be able to use the data as a way to disqualify people from coverage based on the calculated probability of them developing particular disorders. This would be a significant breach of privacy, even for those convicted of crimes.

      Slippery slope to police state?

      It is then a slippery slope towards allowing others to gain access to the databank to learn much more about people than they otherwise would have been able to. It isn’t a large leap to make for people to begin justifying the collection of DNA profiles of all citizens at birth under the guise of using them to help solve crime. Why not establish a voluntary system enabling the government to create a list of people who have not volunteered and then monitor them?

      With an increase in the use of video surveillance on nearly every public corner, and the U.K.’s trend toward inclusion of DNA samples, these actions lend themselves to the actions of a police state with “big brother” watching, and justifying infringements of our most basic rights.


      This two-part discussion outlines the practical and scientific concerns of using the investigative technique known as DNA dragnet. Look here next Friday (March 4) for the concluding column.


      Sherry Baxter practises criminal and civil law on Vancouver Island, as well as provides legal research and litigation services. Reasonable Doubt appears on on Fridays. You can send your questions for the column to its writers at

      A word of caution: You should not act or rely on the information provided in this column. It is not legal advice. To ensure your interests are protected, retain or formally seek advice from a lawyer.