Summary
In a courtroom, jurors and judges can be easily swayed by impressive-sounding DNA evidence and often do not pay attention to, or understand, the statistical evaluation.
As a Criminal Lawyer, knowing when and how to question DNA results is key to maximising their efficacy in your case.
A red flag is seeing low statistical results in isolation—there are multiple factors which can affect these results, and all need to be explored to ensure the evidence is not misleading to the court.
Statistics is a complex branch of mathematics requiring years of study to interpret. In forensic science, for example, it is easy for jurors to confuse Likelihood Ratios (LR: 1000 – moderately strong evidence to support) with odds (1 in 1000 chance) and make poor evaluations on the value of the evidence.
Recently, I had a case that highlighted the dangers of low likelihood ratios generated by laboratories in Australia. In the below excerpt from the DNA report, you can see the low likelihood ratios appearing in isolation.
The Risk of Overevaluating Evidence
When the limitations to the evidence are not disclosed, there is a risk they could be overevaluated by the court. In this case example, there are two key concerns:
1. A Mixed DNA Profile Was Obtained – Who Does This DNA Profile Originate From?
This DNA profile appeared to originate from at least two individuals. While I wasn’t able to include it in the example image above, the complainant is an assumed contributor to this DNA profile. This is reasonable, given the sample has originated from them. However, could this second person be one of the individuals that has generated the low likelihood ratios above? If so, which one?
2. It Is Possible It Could Have Originated from Someone Else
Based on these low results, it is also possible this DNA profile has originated from an individual who has not yet been compared to the DNA profile.
Suggested Action
When you see low statistical results in isolation, it is very important to remember that many other individuals in the population could match the DNA profile, with the likelihood ratio potentially being the result of an adventitious match—a coincidence that they share the same components of DNA.
When presented with a report like this, with no further guidance to assist you in ascertaining who is the contributor, the safest course of action is to seek external validation through a suitably qualified expert witness.
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Jae Gerhard is a Forensic DNA Expert and the Principal Forensic Scientist at Independent Forensic Services, bringing over two decades of frontline experience in forensic biology. She has worked on some of Australia’s most complex and high‑profile cases with the Australian Federal Police and New South Wales Police, as well as international disaster victim identification operations in Bali and Thailand.
Specialising in DNA analysis, STRmix™, biological fluid examination, and bloodstain pattern interpretation, Jae now works independently with criminal defence teams to review and challenge forensic evidence. Her mission is simple: to ensure the science presented in court is accurate, balanced, and truly understood.