The risks posed by mixed DNA evidence to the presumption of innocence in Ireland

DNA is powerful evidence that is popularly celebrated as a forensic “gold standard.”1 Underneath the hyperbole, however, there is a darker side to this impressive biotechnology. Although DNA has a well-earned reputation for its capacity to exonerate, it also has the potential to convict the innocent. Its apparently infallible reputation has been called into question in recent times. Science and the law undoubtedly share a mutual goal—the evaluation of evidence. Yet despite this common aim, lawyers and scientists do not speak the same language. This disconnection has become particularly visible when mixed DNA evidence, which contains the biological profiles of more than two contributors and is commonly found at crime scenes, is relied upon by the prosecution. There are a number of challenges surrounding DNA mixture interpretation in the laboratory which remain unresolved by the scientific community. The presentation of statistics to support DNA mixtures raises further problems: the use of nuanced mathematical tools places significant demands on both the professional and lay fact finder. In addition, the development of the technology to support DNA evidence has grown rapidly. This means that there is still only a relatively small community of sufficiently-qualified scholars worldwide who can provide expert witness testimony when certain complex mixtures are adduced in evidence. This thesis sets out to identify the technical problems with the interpretation of DNA mixtures in the laboratory, including the role played by subjectivity and bias, the types of statistical challenges facing the trier of fact and the potential issues surrounding the use of expert witness testimony in Ireland. The thesis also explores whether the concerns identified pose risks to the presumption of innocence which is underpinned in Article 38.1 of the Irish Constitution. The thesis concludes with a series of recommendations aimed at protecting a suspect’s due process rights when mixed DNA evidence is adduced in the jurisdiction.