DNA evidence is frequently relied upon in sexual assault prosecutions, particularly where DNA is recovered from intimate swabs. However, the presence of DNA alone does not necessarily resolve the critical forensic question before the court: how the DNA was deposited? The District Court decision to exclude DNA evidence in R v Ke provides a clear judicial illustration of this limitation.

In that case, DNA attributed statistically to the accused was detected on vaginal and labial swabs taken from the complainant. The prosecution relied on the presence of this DNA to support an allegation that the accused had performed cunnilingus. However, the scientific evidence could not establish whether the DNA was deposited through direct contact or by secondary transfer.

Helen Roebuck reported and gave oral evidence in R v KE [2019] NSWDC 349; application to exclude DNA.

The Forensic Issue Before the Court

A fact in issue at trial was whether the accused had engaged in oral contact with the complainant’s genital area. Both prosecution and defence experts accepted that the DNA findings could not determine how the DNA came to be present. The evidence was limited to the identification of the accused’s DNA on the swabs.

As Judge Grant observed, even taking the Crown case at its highest, the DNA evidence demonstrated only presence. It did not resolve whether the DNA was deposited directly through the alleged act, or indirectly through secondary transfer.

This distinction was central. The court was not required to decide whether secondary transfer had occurred, only whether the DNA evidence could reliably support the inference the prosecution sought to draw.

Secondary DNA Transfer and Sexual Assault Evidence

Secondary DNA transfer refers to the movement of DNA via an intermediary surface or person rather than through direct contact. In sexual assault cases, this may occur through hands, clothing, bedding, or other forms of indirect contact. The science recognises that such transfer is possible, particularly where low-level DNA is involved.

In R v Ke, neither expert could exclude secondary transfer as a plausible explanation for the DNA findings. As a result, the DNA evidence could not assist the court in resolving the critical issue of alleged sexual activity.

Exclusion of the DNA Evidence Under s 137

The court excluded the DNA evidence under section 137 of the Evidence Act, finding that its probative value was limited and outweighed by the risk of unfair prejudice. The risk lay in the potential for the jury to misuse the DNA evidence – treating presence as proof of the alleged act, despite the scientific inability to support that inference.

The judgment reflects a clear judicial recognition that DNA evidence which cannot answer the activity-level question may mislead, rather than assist, the fact-finding process.

Why R v Ke Remains Significant

R v Ke stands as an important authority on the limits of DNA evidence in sexual assault matters. It confirms that:

• the presence of DNA does not establish how it was deposited
• secondary DNA transfer may remain a live possibility
• DNA evidence may be excluded where it cannot resolve the fact in issue
• strong scientific limitations can outweigh apparent evidentiary strength

The case illustrates that DNA evidence must be assessed not only by what it shows, but by what it cannot show.

Independent Expert Evidence and Activity-Level Evaluation

Forensic laboratory DNA reports typically speak to the “who” , but do not address the activity-level significance of those findings. In R v Ke, the court’s decision turned on the absence of reliable scientific support for the inference advanced, highlighting the importance of independent expert evaluation of transfer mechanisms and evidentiary limits.

Conclusion

R v Ke demonstrates that DNA evidence, even when recovered from intimate swabs, may carry limited probative value where science cannot determine how it was deposited. The case remains a clear example of secondary DNA transfer being a live and unresolved issue, and when DNA evidence can be excluded because its potential for misuse outweighs its evidentiary contribution.

Sexual assault matters frequently rely heavily upon DNA evidence in support of the prosecution case. Careful consideration and presentation of the factors underpinning indirect transfer can assist in determining whether DNA results can be relied upon safely.

Concerned about whether DNA evidence can safely be relied upon in your case?
In sexual assault matters, the DNA findings may carry limited probative value and create a real risk of unfair prejudice.
Helen Roebuck can assist in determining whether exclusion or refusal of the DNA evidence should be considered.