The appraisal of DNA evidence in criminal law

The appraisal of DNA evidence in criminal law

In recent years many initiatives have been taken to improve how criminal law judges interpret DNA evidence. How well is DNA evidence appraised in criminal law courts today?

Most lawyers have good oral skills and are experts in sifting through complex legal texts and jurisprudence. The average lawyer however is often teasingly described as “someone who has no feeling for figures and hard science and so ended up studying law”. On a more serious note though, some understanding of science is no luxury when it comes to criminal law, for example in the field of assessing DNA evidence.

For many years, attention has been paid to the problems surrounding the appraisal of DNA evidence in the criminal courts and discussions have been held on the issue. Empirical research has demonstrated that DNA reports and the manner in which conclusions are arrived at, are not properly understood by parties in criminal proceedings and that mistakes are made in the interpretation of these reports. As a result, in recent years various initiatives have been taken aimed at improving the understanding of criminal courts and consequently also improving the quality of the judgments. In spite of these initiatives, the literature still gives the impression that mistakes are being made in the assessment of evidence. One of the aims of the research I conducted for my thesis in criminal (procedural) law, was to evaluate the quality of how DNA evidence is appraised. To this end, a total of 78 judgments by district courts and courts of appeal were analysed. The quality was assessed by counting how many of these judgments contained errors in reasoning and contained vague or incomplete formulations.

Consider first of all the chance that an animal has four legs, and it is given that this animal is a cow. Then consider the chance of an animal being a cow, given that this animal has four legs. Everyone senses that the chance of the second instance occurring is less than the first chance. In the second case, the animal after all could be a dog, an elephant or a giraffe. If you believe that these chances are equal, you incorrectly reverse the conditions and make a mistake in your reasoning which is referred to as the prosecutor’s fallacy. These types of errors in reasoning - in a more complex form - are also made in the appraisal of DNA evidence and can be seen in a judgment.

In my thesis research I encountered such errors in reasoning in around one third of the judgments analysed. Although this did not necessarily have consequences for the accuracy of the judgment when it comes to the guilt or innocence of the accused, making such an error in reasoning can be problematical. This is certainly true of cases where less evidence is available or in which a complex mixed DNA profile is central to the case. In addition, once more in a third of cases, judges use vague or incomplete formulations. This again is undesirable because it is not clear whether the judge has understood the DNA evidence and has been able to assess it correctly.

Perhaps it is time for criminal (procedural) law education in future, and more often, to take account of the role that science can play in criminal law. Considering the scientific historical background of the Kamerlingh Onnes Building, Leiden Law School would appear to be the most appropriate law school to take the lead to achieve this.

Kiki Twisk received the prize for the best thesis in 2018 for her thesis “DNA-mengprofielen in strafzaken. Een jurisprudentieonderzoek naar de kwaliteit van de waardering van DNA-bewijs” [DNA mixed profiles in criminal trials. Case law research into the quality of the assessment of DNA evidence].


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