DNA Evidence Is Not a Binary Question

In criminal proceedings, DNA evidence is often viewed in binary terms: either it is admitted and supports the prosecution case, or it is excluded and removed from consideration. In practice, the treatment of DNA evidence is far more nuanced.

DNA is inherently persuasive. Its presence almost always lends some weight to the prosecution narrative, even where the scientific limitations are significant. For this reason, defence practitioners may instinctively prefer exclusion wherever possible. However, exclusion is not always achievable, and it is not always strategically desirable.

There are circumstances in which the weakness of the DNA evidence itself—when properly understood and explained—can form a central part of the defence case.

---

The Role of Secondary DNA Transfer

Secondary DNA transfer refers to the movement of DNA via an intermediary, rather than through direct contact between a person and an item. This may occur through other individuals, shared environments, or frequently handled surfaces.

Modern forensic science recognises that DNA can be transferred in multiple ways and that the presence of a DNA profile does not, of itself, establish contact. As a result, DNA findings may be consistent with a range of explanations, only some of which involve the alleged activity.

Where those alternative explanations cannot be excluded, the probative value of the DNA evidence may be limited.

---

DNA Admitted, but Given Little Weight

In R v Kabbout [2020] NSWDC 707, DNA attributable statistically to the accused was identified within a mixed DNA profile recovered from the handgrip of a handgun. The DNA evidence was admitted at trial.

However, the trial judge declined to place significant weight on the finding. In assessing the evidence, the court accepted that there were multiple realistic possibilities by which the accused’s DNA could have been transferred to the firearm without the accused ever having touched it directly.

Of particular significance was evidence that a close family member of the accused had been present in the vehicle where the firearm was located, and that there was regular contact between the accused, that person, and shared environments within the vehicle. These circumstances gave rise to plausible indirect transfer pathways that could not be excluded on the science.

The judge concluded that the DNA evidence did not reliably establish contact with the firearm and could not support the inference sought by the prosecution. The matter ultimately resulted in a not guilty verdict.

---

Weight Versus Admissibility

This case illustrates an important distinction. DNA evidence does not need to be excluded in order to be neutralised. Courts are prepared to admit DNA evidence while recognising that it may carry limited weight where transfer explanations remain unresolved.

From a defence perspective, this raises a strategic consideration. While DNA evidence is often prejudicial, excluding it may remove the very weakness upon which the defence relies. In some matters, the inability of the DNA evidence to answer the critical question of contact or activity may be the most tangible feature undermining the prosecution narrative.

The assessment of DNA evidence is therefore not simply about whether it comes before the jury, but how it is framed, understood, and weighed.

Excluding DNA evidence by application warrants careful matter specific consideration.

---

The Importance of Contextual Evaluation

DNA evidence does not exist in isolation. Its significance depends on context, including who had access to the environment, how items were handled, and whether there are realistic pathways for indirect transfer.

Where such pathways exist, DNA findings may be scientifically incapable of resolving the issue the court is asked to determine. Identifying and articulating these limitations requires careful evaluation of both the forensic results and the surrounding circumstances.

---

Strategic Implications for Defence Cases

The increasing recognition of secondary DNA transfer has practical consequences for defence strategy. Decisions about whether to seek exclusion, or instead to expose the limits of the DNA evidence, must be made on a case-by-case basis.

An expert who understands both the science and the forensic context can assist in determining whether DNA evidence should be challenged on admissibility grounds, or whether its weakness is better left visible and explored before the trier of fact.

---

Conclusion

The presence of DNA does not equate to proof of contact. Nor does the admission of DNA evidence require that it be accepted as probative of the alleged activity. Where multiple transfer pathways remain plausible, DNA evidence may carry little weight, even when it is admitted.

Cases involving secondary DNA transfer demonstrate that the real forensic question is often not whether DNA evidence should be excluded, but whether it can safely support the inference being advanced. Understanding that distinction is now central to modern criminal DNA casework.

Concerned about how DNA evidence is being relied upon in your case?
Independent forensic review can assist in clarifying whether DNA findings genuinely support the alleged activity, or whether indirect transfer and contextual factors significantly limit their probative value.

Helen Roebuck, forensic DNA expert, assists criminal lawyers nationwide with independent DNA evaluation, expert advice, and court evidence.