DNA evidence in sexual assault prosecutions is often presented as conclusive and highly compelling evidence, but there are serious scientific and legal limitations. Defence lawyers should be alert to the following issues when preparing or challenging such cases.
1. No Spermatozoa Counts in Reports
Forensic reports usually record only the presence of spermatozoa. They do not disclose numbers, which can be highly relevant and may or may not record the presence of tails. A handful of sperm cells may be consistent with historic or indirect transfer, not recent penetration.
2. Timing is Ignored
Spermatozoa can persist for days – sometimes longer. Reports rarely assess whether findings align with the complainant’s alleged timeframe. Without context, juries may wrongly assume presence equals recent activity.
3. Seminal Fluid Testing Not Always Conducted
In some cases, testing for the presence of seminal fluid is not performed. This omission is critical, as the detection (or absence) of seminal fluid may directly affect the interpretation of whether sexual activity occurred. Without this analysis, the evidence may appear stronger than it actually is.
4. Risk of Innocent Transfer
Sperm and semen can transfer via laundry, bedding, or indirect contact. Such possibilities must be put before the jury as reasonable alternative explanations.
5. Sampling and Contamination Risks
Swabbing may push external material into the vagina, creating the appearance of penetration. This occurs 100% of the time if a speculum is used. Contamination during collection, packaging, or laboratory analysis is a recognised risk. In controlled testing, 52% of samples collected showed transfer between intimate sampling sites. Spermatozoa and / DNA in intimate locations may not always mean penetration as occurred.
6. Why Independent Review is Critical
An independent forensic DNA expert can assist in identifying the above limitations and more. Engage an expert early. Timely advice can shape trial strategy, assist in cross-examination, and prevent juries from being misled.
Key Takeaway: DNA evidence in sexual assault matters is not infallible although often appears as highly compelling. Defence lawyers should never accept a DNA report at face value. Independent forensic review – sought early – is often decisive in ensuring a fair trial.
Author Bio
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.
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.
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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.