Understanding a Common Misinterpretation in Sexual Assault Cases
In sexual assault prosecutions, DNA evidence often carries significant weight — particularly where a report refers to semen detection alongside a DNA likelihood ratio that includes the accused. It is not uncommon for this combination to be interpreted as meaning the accused’s DNA is attributable to semen identified on an exhibit.
Scientifically, that conclusion does not automatically follow.
This misunderstanding can materially alter how the evidence is evaluated, plea considerations, how the narrative is framed at trial, and how probative value is assessed. A closer look at how semen is identified — and how DNA is actually extracted — reveals why caution is required
The difference between detecting semen and attributing DNA to it
When a forensic report refers to semen, it may be based on one or more different forms of testing. These vary in reliability and in what they can robustly establish.
Presumptive tests are not confirmatory
Initial screening for semen often involves presumptive testing, which is designed to indicate whether semen may be present. These tests are sensitive but not specific. They can react to other biological substances and therefore cannot conclusively establish the presence of semen.
While useful as a screening tool, presumptive results should not be treated as proof.
Confirmatory testing has limits
More specific methods exist, including tests designed to detect semen-specific proteins. Even these tests, however, do not directly link a detected semen component to a particular DNA profile. They confirm that semen is present somewhere on the item tested — not whose semen it is.
Another common method involves the identification of sperm cells under a microscope. While microscopy can provide stronger support for semen presence, it is not always conclusive. Factors such as sample degradation, low sperm count, or mixed biological material can complicate interpretation.
Why semen detection does not equal DNA attribution
Even where semen is reliably identified, an additional scientific step is required before DNA can be said to originate from that semen: differential extraction.
Differential extraction is a laboratory method used to separate sperm cells from other cellular material (such as skin cells). In theory, this allows analysts to isolate sperm-derived DNA. In practice, the process is not always successful and carries recognised risks, including:
- incomplete separation of sperm and non-sperm cells
- loss of material during processing
- mixed DNA results despite attempted separation
- low-level DNA profiles that do not clearly indicate origin
As a result, the DNA profile ultimately obtained may not be attributable to semen at all. It may instead arise from skin cells or other biological material present on the item.
Why TIME SINCE INTERCOURSE (TSI) FURTHER COMPLICATES
Time since intercourse may be a probative consideration in matters involving internal ejaculation. Through the careful observation of the number of sperm heads, and the presence or absence of sperm tails, one can scientifically formulate certain guidance on the realistically possible timeframe of deposition.
Further, certain evidentiary matters require consideration as to the possibility of semen contributions from more than one person. This commonly arises where a sexual assault with ejaculation is alleged, whilst there is also the possibility that the complainant has engaged in recent prior intercourse (ie with their partner).
The ability to attribute DNA to the sperm critically underpins any time since intercourse evaluation.
Why this distinction matters in court
Where semen is identified and a DNA profile includes the accused, there is a natural tendency to link the two findings. However, unless the laboratory results specifically demonstrate that the DNA profile originated from sperm cells, that connection remains an inference — not a scientific conclusion.
This distinction can significantly change the evidentiary narrative:
- DNA attributed to semen may suggest recent sexual activity
- DNA from skin cells or indirect transfer may support alternative explanations
- mixed-source DNA complicates assumptions about timing and activity
Understanding these differences is critical when evaluating probative value.
A common source of misunderstanding
Many forensic reports present findings in a structured, technical format that does not always explain what can and what cannot be concluded. The absence of explicit limitations can lead to assumptions that extend beyond what the science supports.
Importantly:
- the presence of semen does not automatically identify a contributor
- the presence of a DNA profile does not automatically indicate biological source
- likelihood ratios address whose DNA may be present, not how it was deposited
These distinctions are essential for fair interpretation.
The role of independent forensic review
Independent review of biological fluid testing and DNA interpretation can help clarify:
- whether semen identification was presumptive or confirmatory
- whether sperm cells were definitively observed
- whether differential extraction was attempted and successful
- whether the DNA profile can or cannot be reasonably attributed to semen
- what alternative explanations remain scientifically plausible
In some cases, this analysis can significantly alter how the DNA evidence should be understood.
Final observation
DNA evidence is often powerful, but its meaning depends on careful scientific boundaries. When semen detection and DNA findings are considered together without examining how they relate, the evidentiary narrative can shift beyond what the science supports.
A clear understanding of these distinctions ensures that conclusions drawn in court remain aligned with defensible forensic principles — and that the weight given to DNA evidence reflects its true probative value.
Helen Roebuck is a strongly credentialed and highly regarded forensic DNA expert, trusted within both the legal and scientific communities. She is the most prolific and judgment-published independent DNA expert witness in Australia, with her evidence extensively referenced across various jurisdictions.
Concerned about attribution of DNA to Semen in your case?
Our independent review can clarify whether any identification of semen is scientifically robust and whether the DNA profile is or is not reasonably attributable to semen.
Contact Roebuck Forensics to discuss an independent forensic DNA case review or expert witness support.