Birgitte Tengs case: Analysis and the wider implications for evaluation of DNA evidence given activities

Abstract

The Birgitte Tengs case was a high-profile investigation into an historic 1995 murder in Karmøy municipality, South East Norway. Rapidly mutating Y-STR analysis was carried out to identify a possible offender. The work was carried out over several years and involved a collaboration of four international laboratories. Based on the DNA evidence, in February, 2023, the defendant was convicted but later that year, he was exonerated on appeal. New investigations, resulting in new evidence, were carried out before the appeal. The new evidence gave alternative explanations for the presence of Y-STR in a stain that the conviction in the first trial relied heavily on. Despite advanced techniques such as massively parallel sequencing and an extensive genealogy study encompassing Norway and beyond, the judgment revealed systemic issues, which included confirmation bias and reverse burden of proof. The case highlighted the critical importance of interpreting results according to the level at which propositions were made (e.g. sub-source or activity), as failure to do so could contribute to potential miscarriages of justice. Here, it is proposed that when there is no meaningful (specific) defence activity proposition identified, proxy experiments designed to maximise the probability of evidence supporting the defence alternative, will play an important role. Bayesian networks are useful methods to provide courts with lists of exhaustive methods of DNA transfer and has information illustrative properties. However, such networks can only help in actual probability computations if they can be populated with informative probabilities, and this can only happen if there is agreement on an experimental design to follow. This paper examines the forensic methods, trial findings, and appeal court verdict, identifying areas where collaboration between scientist and court could improve decision-making. It proposes experimental designs and frameworks for enhancing activity-level evaluations, offering insights into minimising miscarriages of justice and better integrating forensic genetics into legal systems.

Keywords: Bayesian networks; Birgitte Tengs; Confirmation bias; Hierarchy of propositions; Miscarriage of justice; Y-STR profiles.