DNA transfer is now a familiar concept in criminal proceedings. Most practitioners recognise that the presence of DNA does not, of itself, establish how it was deposited. The proposition that science cannot conclusively determine the pathway of transfer is widely accepted and often forms part of defence thinking as trial approaches.
In practice, this understanding frequently leads to the development of one or more hypothetical explanations for how DNA may have come to be where it was found.
That approach is understandable. It reflects the reality that multiple pathways are often scientifically possible. But what is possible is not always persuasive, and in the courtroom the distinction between “possible” and “reasonably plausible” becomes critical.
The role of hypotheticals
Hypothetical transfer scenarios are commonly used to demonstrate uncertainty. A lawyer may suggest that DNA could have arrived via indirect contact, secondary transfer, shared environments, or routine household activity. Each of those pathways may be scientifically valid in the abstract.
The difficulty arises when multiple hypotheticals are layered on top of one another.
Individually, each scenario may be possible. Collectively, they can begin to feel constructed. As layers accumulate, the narrative risks shifting from grounded uncertainty to speculation. Jurors do not assess each hypothetical in isolation; they assess the overall plausibility of what they are hearing.
Extensive layers of doubt, when built upon one another, can start to appear fanciful rather than persuasive.
How complexity enters the courtroom
At trial, a prosecution expert will rarely claim to know precisely how DNA was deposited. Instead, the expert will acknowledge uncertainty while introducing factors said to influence transfer. These often include:
- friction
- duration of contact
- quantity of DNA recovered
- persistence
- surface type
- background DNA levels
- DNA contamination
- findings from studies or validation work
Each point may appear measured. But together they form a structured explanation that sounds grounded and coherent towards the likelihood of direct transfer. The expert does not need to assert that a direct transfer occurred; the accumulation of factors can subtly guide the jury toward that conclusion.
In response, defence counsel may return to hypotheticals. More scenarios are suggested. More pathways are introduced. More possibilities are explored.
The result is often the opposite of what was intended.
As complexity increases, the prosecution narrative can appear increasingly anchored in real-world conditions, while the defence alternatives begin to sound abstract. The discussion shifts away from the central issue and into competing theoretical constructs.
When “possible” becomes implausible
Courts and juries do not evaluate transfer by asking whether a pathway is scientifically possible. They evaluate whether it is plausible in the circumstances of the case.
This is where difficulty arises.
Hypotheticals that are not grounded in the uncontested background of the matter risk drifting into territory that feels disconnected from the lived reality of the events under consideration. A series of technically possible pathways may exist, but if they are not anchored to the facts the jury already accepts, they can lose persuasive force.
The issue is not whether a hypothetical could occur. The issue is whether it makes sense in the context of the case.
For example, placing a handshake before the court in a matter that does not involve handshaking may introduce a scientifically valid pathway, but one that feels unrelated to the circumstances being tried. The jury is left weighing an abstract construct against a narrative rooted in the events before them.
Why put the handshake before the court when the matter does not involve handshaking?
Grounded alternatives and the control of evidence
Alternative explanations do not need to amount to a defence “version.” Nor is the defence required to prove how DNA came to be deposited.
What often proves more effective is the use of pathways grounded in the uncontested background of the case itself.
Shared household spaces, routine handling of objects, laundering, repeated contact with common surfaces — these are not speculative constructs. They are part of the lived environment in which the allegation is framed. When raised carefully, they can narrow the discussion to variables that are genuinely relevant.
Grounded alternatives limit how far expert testimony can wander.
Rather than inviting a broad discussion of theoretical transfer mechanisms, they focus the forensic conversation on the specific conditions of the matter. Jurors are then asked to consider what is plausible in that environment, not what is possible in an abstract scientific sense.
This distinction is subtle but powerful.
Where transfer strategy is really set
The simplicity or complexity of transfer evidence at trial is dangerous when determined in the witness box. It should be shaped beforehand — by how the issue is framed, what is addressed in reporting, and how the boundaries of scientific interpretation are established.
When those boundaries are defined early, expert testimony is more likely to remain anchored to the circumstances of the case. When they are not, the discussion can expand rapidly into layers of variables, research references and technical qualifiers that are difficult to control once introduced.
For practitioners, the key question is not whether multiple transfer pathways can be imagined, but how those pathways will be received when tested against plausibility.
What is technically possible may not always assist. What is grounded in the uncontested reality of the case often carries greater weight.
Where DNA transfer forms a central part of the forensic evidence, early consideration of how alternatives will be framed can materially influence how the issue unfolds at trial. Independent forensic input does not replace legal strategy. It assists in defining the scientific ground on which that strategy operates — including what can properly be asserted, what remains uncertain, and how transfer evidence is likely to be interpreted in the context of the case.
Concerned about how DNA transfer alternatives will be received at trial?
What is scientifically “possible” can quickly become implausible when layered hypotheticals drift away from the facts of the case.
Helen Roebuck can provide an independent expert review assist in identifying grounded transfer pathways and clarifying how the evidence is likely to be interpreted in the courtroom