The Politics of Truth in the Courtroom: Knowing about DNA Evidence

24 June 2011

The labyrinthine maze of Bayesian statistical reasoning is hardly something that an average jury member could be expected to successfully navigate. Yet this is exactly what is expected when forensic DNA evidence is presented in the courtroom.

Article courtesy of The Royal Society of New Zealand (

The labyrinthine maze of Bayesian statistical reasoning is hardly something that an average jury member could be expected to successfully navigate. Yet this is exactly what is expected when forensic DNA evidence is presented in the courtroom. The baffling nature of the evidence, which is read out only once, would likely intimidate even the most educated members of our communities.

In New Zealand, forensic scientists analyse DNA samples at the request of the Police. A sample may be taken from a crime scene, the DNA extracted and then compared to a ‘reference sample’ taken from a known individual to establish whether or not the DNA could have come from the same source.

Scientists provide their expert findings on this question. This may sound simple enough, however, this apparent simplicity is highly misleading. Where the scientific meaning of DNA evidence becomes a pawn in the adversarial legal process of prosecution and defence, it then becomes imperative to track how the probabilistic nature of evidence fares in the politics that structure this encounter. After all, the ‘safety’ of the jury’s judgement is at the heart of our criminal justice process.

A research team led by Professor Victoria Grace from the University of Canterbury and Professor Gerald Midgley (formerly ESR, now University of Hull), and including Johanna Veth from ESR and Annabel Ahuriri-Driscoll from the University of Canterbury, has been investigating what various interested parties within the criminal justice system actually make of DNA evidence, what they think it means and how they evaluate it.

Qualitative research revealed some notable differences in the way forensic DNA evidence is interpreted across a range of professional groups and members of the lay public. Professionals were asked to respond to a prototype of evidence as presented in New Zealand courts. Forensic scientists were clear about the probabilistic nature of the evidence, and understood that the claim of correspondence between samples is ultimately a judgement or opinion based on an assessment of the evidence (which may be extremely strong). However, this was not the predominant understanding for other professional groups nor for the lay public.

Police detectives, scene of crime officers, and crown prosecutors were more inclined to view the evidence as a simple matter of fact. There was a clear pattern of interpreting the scientist’s opinion about the interpretation of the statistical ‘likelihood ratio’ as one representing certain truth that the samples are from the same source. Case closed.

Members of the lay public discussed a crime scenario in focus groups, so were presented with the same statement of evidence but in a different context. Here a clear pattern of response indicated that their expectation was one of certainty from the science. Yet when the evidence was presented in terms of probability (even using unfathomably low ratios like one in a million, million), their confidence in this certainty was shaken.

This led to significant problems in interpreting the evidence. Although there was a sub-group for whom the lack of absolute certainty was unproblematic, the dominant responses ranged from baffled and confused to hazarding wildly erroneous interpretations. This in turn undermined focus group members’ ability to integrate the evidence into what would be a judgement about guilt or otherwise in an actual trial.

Analysis of the juxtaposition of these responses alerted researchers to the structural positioning in which jury members might be placed. That is, being vulnerable to the influence of legal adversarial rhetoric guiding the assessment of scientific evidence. Also, when the legal defence is not equipped with the knowledge and resources to sustain a counter argument to the prosecution about the meaning of the DNA evidence, then the result potentially diminishes the contribution of science to justice.

The research team rejects the view that debate about the meaning of DNA evidence should be removed from the courtroom, as some commentators suggest. They argue that to do so would present the appearance to jurors of removing the ‘politics’ from knowledge, and science would falsely resume its popularised mantle of truth and certainty.

Instead, improvements in the wording of evidence and greater education in the critical nature of science and the meaning of the statistical evidence presented in court is suggested for members of non-scientific professional groups, including lawyers. Jurors also need information on the meaning of probability statements, prior to trial.

Without such education and improved understanding there is a risk that evidence is made to say what it does not and cannot say, thus potentially compromising justice. While the researchers would not expect that trials involving DNA evidence would necessarily be judged differently in terms of outcome, they do advocate a process whereby juries are strengthened in their assigned task of providing independent judgement.

  • Forensic DNA Evidence on Trial: Science and Uncertainty in the Courtroom by Victoria Grace, Gerald Midgley, Johanna Veth and Annabel Ahuriri-Driscoll (2011) has just been published by Emergent Publications, and presents these research findings in detail.


For more information please contact:
Professor Victoria Grace
School of Sociology and Anthropology
University of Canterbury
Ph: (03) 364 2692

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