When the Evidence Changes After Collection
Criminal lawyers are taught to scrutinise how DNA evidence is collected at a crime scene.
Far less attention is paid to what happens after the exhibit is sealed.
Yet a growing body of forensic research shows that DNA evidence can change inside its own packaging – during transport, storage, and routine handling – without any further human contact.
For lawyers relying on DNA to prove or rebut allegations, that should give pause.
DNA Does Not Stay Where It Was Deposited
A landmark forensic study examined what happens to DNA once an exhibit is packaged and moved in a manner consistent with routine police handling. The findings were stark.
Significant quantities of DNA were shown to:
- – transfer from the original deposit site to other areas of the same item
- – transfer onto the inside of the packaging
- – transfer between multiple items packaged together
In some experiments, more than half of the recoverable DNA left the original deposit site and relocated elsewhere during ordinary transport and storage conditions.
No additional handling.
No laboratory contamination.
Just packaging, movement, and time.
Why This Matters in Criminal Cases
DNA results are often presented as if the detected profile reflects where the DNA was originally deposited.
The science does not support that assumption.
Packaging-mediated transfer can result in DNA appearing:
- – on areas of an item that were never directly contacted
- – on secondary items packaged together
- – in locations later relied upon to infer how an offence occurred
In some scenarios examined in the research, DNA originating from one item became detectable on another item to the point that mixtures were dominated by the “wrong” contributor.
Once that happens, interpretation becomes complex – and potentially misleading.
The Problem Is Not Just Transfer – It’s Interpretation
Forensic DNA reporting typically focuses on whose DNA was detected.
But courts are increasingly concerned with how and when that DNA may have been deposited.
Packaging-related transfer blurs that distinction.
If DNA can relocate within packaging:
- – the activity-level meaning of the result may be overstated
- – apparent “contact” may never have occurred
- – the probative value of the evidence may be materially weaker than the statistic suggests
A strong likelihood ratio at the source level does not rescue a weak or unexamined activity-level proposition.
Common Casework Scenarios at Risk
The research demonstrated significant DNA redistribution in scenarios that closely resemble everyday casework, including:
- – clothing items packaged together
- – cigarette butts stored in a single envelope
- – gloves or tools co-packaged in bags
- – swabs losing DNA to their containers before testing
In several instances, DNA was detected on areas of an exhibit that had never been in direct contact with the biological source.
From a legal perspective, this creates fertile ground for misinterpretation if the issue is not identified and addressed.
Why These Issues Are Often Missed
Packaging details rarely appear explicitly in forensic reports.
Even when available, the laboratory notes may not record:
- – how tightly an item was packaged
- – whether multiple exhibits shared packaging
- – whether internal packaging surfaces were examined
Without targeted review, lawyers may never be told that alternative explanations exist – despite being scientifically plausible.
This Is Where Expert Review Becomes Critical
Packaging-mediated DNA transfer does not mean the evidence is “wrong”.
It means the evidence is conditional, context-dependent, and vulnerable to overstatement if interpreted simplistically.
Identifying these issues requires:
- – understanding DNA transfer and persistence science
- – careful review of collection and packaging records
- – activity-level evaluation, not just source attribution
- – awareness of how courts assess probative value and unfair prejudice
These are not matters that can be resolved by statistics alone.
The Quiet Question Every Lawyer Should Ask
Before accepting what a DNA result appears to show, ask:
Could this DNA have moved after collection?
If the answer is possibly, the evidentiary weight may be very different from how it is being presented.
And that difference can be case-critical.
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 jurisdiction.
Concerned about contamination or DNA transfer in your case?
If DNA evidence is central to the matter, an independent review can clarify whether contamination, secondary transfer, or low-level DNA interpretation may have affected the conclusions.
Contact Roebuck Forensics to discuss an independent forensic DNA case review or expert witness support.