Apply science carefully in court

The power of DNA expert evidence to inculpate is unquestionable.

The authority of DNA mandates significant exculpatory efforts in order to appropriately inculpate.

DNA evidence is relied upon within the Australian criminal justice system to a greater extent, and with more force than any other physical evidence.

“Like all evidence DNA can be used, misused and abused.” 1

At its infancy in the 1990s and early 2000s, DNA expert evidence was challenged extensively on the basis of its very foundation as a forensic science tool. Over the decades, DNA has become widely accepted as reliable and highly effective evidence before the courts.

The legal profession has taken to accepting and absorbing the science with vigour. The comprehension of practitioners and the judiciary as to the broader concepts of DNA is well established. For all its glorious achievement, the science of DNA is a continuum, correctly seeking to learn through the past and fold that learning into our current theory and process.


“It is the gold standard of scientific evidence.” 2

DNA activity level evidence

DNA evidence can be considered in terms of “identity” and “activity”, with the latter referring to the “how” and “when”.  The extensive scientific development and advancement over preceding decades is largely attributable to identity, with further learning and outcomes yet to be realised with activity level (including DNA transfer).

“The concepts of primary, secondary and tertiary DNA transfer have been widely reported in scientific journals and revealed in cases of wrongful conviction.” 2

The justice system turns to science with increased expectation as to how and when DNA was deposited. This is in part a natural progression of seeking further answers, whilst also flowing from increased testing sensitivity, which has delivered a greater volume of profiles with an increased number of contributors within those profiles.

At this point, we are able to consider certain factors as they relate to activity level, though great care should be taken with generalities or assumptions. What can be appropriately considered scientifically, is at times distinctly different to what might seem logical, and indeed differ from what may at times be inferred in the courtroom more broadly.

Science rightly continues to experiment, research and advance. At the same time, great care must be exercised when seeking to apply science within the courts.


Activity level – Changes in court

As pressure mounts for science to offer greater certainty with DNA transfer, we are seeing more concerted efforts to test activity level evidence in the courtroom. Hugin Expert is a proprietary computer software suite which utilises Bayesian Networks in the application of mathematical algorithms to weigh data, statistics and risk, seeking to determine likelihoods and assist in decision making. The software markets itself predominantly in fields of financial credit, risk and fraud.

Utilisation of the software seeks to evaluate two propositions, for example the scenario where an item within a complex crime scene, is said to have had DNA deposited through touch. The software is returning likelihood ratios such as 500,000 x more likely, when numbers of this magnitude are previously unreported.

“It is submitted that the introduction of activity level DNA evidence into the courts has occurred prior to experts being properly prepared for such questioning, and without formal authorizations or validation. The courts should consider the weighting of DNA transfer evidence with great caution, and beware of ‘evidence creep’. Activity level DNA evidence is not at the same level of scientific maturity as identity level evidence.

The challenge for the criminal justice system is how to maintain the use of highly probative DNA evidence, while also addressing the complexities associated with the use of this evidence in the courtroom”.  (Wrongful conviction law review – 2020) 2

Activity level – Tip for practitioners

The discussion of activity level in the courtroom is not in all circumstances as clear cut as it should be. Verbal qualifiers are frequently forwarded such that something is “more likely” or “possible” etc. This may come from the crown expert evidence, or simply come to light orally during proceedings. It is vital that verbal qualifiers are only used with the support of a numeric qualifier. Numerical qualifiers need to be substantiated through empirical studies.

DNA evidence before the courts should not be assigned a verbal qualifier without a robust numerical qualifier in support.


1 Andrew Haesler – Dealing with DNA in court, its use and misuse
2 Weathered, L., Wright, K., & Chaseling, J. (2020). Dealing with DNA Evidence in the Courtroom: A Plain English Review of Current Issues with Identification, Mixture and Activity Level Evidence. The Wrongful Conviction Law Review

Author Helen Roebuck is a forensic biologist and DNA expert.

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