Challenging Expert Evidence – It’s in (Y)our DNA
The article Challenging Expert Evidence – It’s in (Y)our DNA is written by Doogue + George Defence Lawyers.
Doogue + George are experts in criminal law and have been involved in thousands of criminal matters and defended clients in hundreds of jury trials and thousands of other criminal cases. Our experienced lawyers have unparalleled experience in criminal law.
DNA is considered to be one of the most robust forensic sciences. It can serve as convincing evidence of a person’s innocence, however, the gold standard attributed to its accuracy can be misleading where DNA evidence is contaminated or otherwise compromised. The inclination towards assuming its accuracy carries with it the risk of miscarriages of justice.
Our genetic code is signified through a sequence of letters and it is the order of these letters which makes us who we are. When this unique sequence is taken from a crime scene and the DNA profile is compared to that of a suspect, it can be compelling evidence of guilt.
However, taken for inculpatory purposes in isolation of other evidence has led to wrongful convictions.1 In R v Jama (2009), for example, the sole evidence was contaminated DNA evidence, which led barrister Saul Holt QC to warn:
To keep up with the volume of modern DNA testing, automated platforms capable of running hundreds of thousands of iterations in a matter of minutes has expedited the process for DNA testing.
It means DNA testing has become more accessible, efficient and cheaper to procure. However, with increased availability and reliance it can be easy to assume DNA’s apparent certainty without acknowledging how it can be misused and misapplied.3
Your lawyer can challenge the evidence where it is more prejudicial than probative. Many assumptions are made in procuring DNA evidence and in decision making about that evidence. There is a strong body of research that demonstrates jurors give excessive weight to certain forms of statistical evidence.
Challenging DNA Evidence
DNA evidence is often expressed as likelihood ratios which signify the chance of seeing two matching profiles by comparing the probability of the prosecution and defence hypotheses and how they might explain the evidence.
The ratio relies on a DNA scientist to input what they deem are a reasonable number of contributors to the profile, and this subjective determination can skew results. Where a wrong number of contributors are inputted, it can be a great source of inaccuracy amongst other evidence yet be masqueraded as persuasive evidence of guilt.
A persistent danger is false inclusions where another person, such as a contaminator, is included in the analysis. There are no disclaimers which recognise the rates of false inclusions, either in the report or in the appendix to that report. Without these limitations and potential for errors being recognised, the ability for DNA evidence to be assessed in light of these risks is compromised.
By looking to technical aspects such as the production of multiple results, the order of magnitude of the results and the likelihood ratio, your lawyer can understand the limitations associated with the evidence, in addition to spotting issues in the way the forensic evidence is derived.
They can tease these issues out in the witness box and question how the expert came to their conclusions. In some cases, for example, multiple tests have been run with different numbers of contributors and one of these results just wasn’t disclosed.
It is important to have an expert lawyer who can effectively question this highly complex scientific evidence to ensure that the full picture of evidence is not lost on a jury.
The shortcomings of DNA evidence can be identified and challenged before it is able to be presented to a jury to make findings of fact on the basis of compromised evidence, whether by contamination, motivational bias, subjective determinations of the forensic scientist or otherwise.
The assumptions that may lead a jury to view DNA evidence favourably or those relied upon by the forensic expert ought to be challenged. This can go some way to address the invisibility of how these assumptions play out within the new “black box” – a system in which inputs and outputs can be observed, but how the outputs are created is unknown. If DNA evidence is as convincing as the gold standard attributed to it, it can rise to the challenge.
 R v Jama (Unreported, Supreme Court of Victoria, Court of Appeal, 2009).
 DNA Symposium, Lifting the Veil on DNA Evidence: What Do the Statistics Really Mean? (Brisbane, 30 June 2017) [DNA Symposium].
 Andrew Haesler SC, ‘Dealing with DNA in Court: its use and misuse’ (2008) 8 (1) Judicial Review
 Evidence Act 2008 (Vic) s 137.
Date Published: 11 May 2022