What Criminal Lawyers Should Know — and Why IT is dangerous
In criminal cases, lawyers may find themselves familiar with the phrase “touch DNA.”
In scientific research and literature, the term might be used to describe DNA originating from skin cells left behind after contact by touch with an object, typically a DNA free plate.
In court practice, it may be casually carried over, erroneously implying that a specific DNA profile is in and of itself consistent with direct contact by touching.
What must be understood, is that the term “touch DNA” is invalid, unless the sampled DNA profile is factually known to be the result of touching, which can never be known in casework.
Whilst the term may still be found somewhat in research, within forensic scientific casework and reporting, the term “touch DNA” is now considered outdated. It oversimplifies DNA transfer considerations and encourages dangerous conclusions that extend beyond what can be scientifically supported,.
For criminal lawyers, understanding these limitations is critical — particularly where the court may otherwise treat DNA presence as indicative of direct contact.
What lawyers are told “touch DNA” means
Traditionally, “touch DNA” has been used to describe DNA profiles obtained from:
- skin cells shed through casual contact
- objects handled briefly
- surfaces exposed to indirect contact
The phrase implies that DNA detected on an item results from physical touch by a specific individual. While this may sometimes be true, forensic research shows that the pathways through which DNA can be deposited are far more complex.
Why the term is scientifically outdated
Modern forensic biology recognises that DNA transfer does not operate in a simple “touch equals deposit” model. DNA can be transferred in multiple ways, including:
- direct contact
- secondary transfer (via another person or object)
- Indirect transfer (meaning more than one intermediary transfer step)
- environmental movement
- background DNA accumulation
Because of these pathways, scientists increasingly avoid the term “touch DNA” in favour of more accurate descriptions such as trace DNA or low-level DNA.
The concern is that “touch DNA” can imply certainty about how DNA was deposited, when in reality, the presence of DNA rarely reveals the mechanism of transfer.
The risk in criminal cases
In criminal casework, the presence of DNA on clothing, skin, or objects may be interpreted as evidence of contact between individuals. Without careful scientific framing, this can evolve into an assumption that:
- the DNA must result from direct interaction
- the interaction occurred during the alleged incident
- alternative explanations are unlikely
However, research demonstrates that:
- DNA can transfer without direct contact
- low-level DNA may persist over time
- background DNA may be present before an alleged event
These factors complicate attempts to draw conclusions about activity based solely on DNA presence.
Why this is difficult to explain without expert evidence
Courts often rely on expert testimony to understand scientific nuance. Where only one scientific interpretation is presented, the limitations of DNA transfer may not be fully articulated.
Criminal lawyers may find it challenging to persuade the court that:
- DNA presence does not equal proof of contact
- secondary DNA transfer (indirect transfer) is scientifically plausible
- low-level DNA findings should be interpreted cautiously
Without expert evidence, these arguments may appear speculative rather than grounded in established research.
The role of independent expert review
Independent forensic review can assist by:
- explaining scientifically recognised transfer mechanisms
- identifying whether alternative pathways remain plausible
- clarifying the limits of inference
- providing a balanced framework for evaluating probative value
This helps ensure that DNA evidence is interpreted within scientifically defensible boundaries, rather than through outdated terminology.
A broader issue DNA transfer in criminal casework
Criminal matters often turn on competing narratives of contact. Where DNA evidence is central, its interpretation can significantly influence perceived credibility. Ensuring that courts understand both the strengths and limits of DNA findings is essential to maintaining evidentiary fairness.
Final observation
DNA evidence is powerful, but its meaning depends on context. The term “touch DNA” may suggest a simplicity that forensic science no longer supports. Recognising the complexity of DNA transfer allows lawyers to approach such evidence with the caution it requires.
Independent expert analysis can assist in presenting these limitations clearly, ensuring that scientific conclusions remain aligned with what the evidence can reliably establish.
Helen Roebuck is an independent forensic DNA expert specialising in DNA interpretation, transfer, contamination, and evidentiary limits. She assists criminal lawyers across Australian jurisdictions with assessing whether DNA evidence can safely be relied upon, how it should be clarified, and whether it may be vulnerable to exclusion or refusal.
Concerned about how “touch DNA” is being interpreted in your case?
Where DNA is assumed to reflect direct contact, transfer mechanisms and low-level findings may be overlooked, creating a risk that the evidence is given more weight than it can reliably support.
Roebuck Forensics can assist in clarifying the limits of DNA interpretation and whether the findings can safely be relied upon at trial.