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Forensic DNA evidence can be broadly grouped into two distinct categories, being source (who) and activity (how).

In 2024, over 95% of DNA matters reaching court are confined exclusively to source (who), meaning there has been no scientific evaluation as to activity (how). When the expert reporting relates only to source, the discussion at hearing of DNA – TPPR should be intrinsically general, and should never be allowed to infer that there is any scientific understanding of DNA – TPPR factors upon the DNA results themselves.

The prejudice of DNA – TPPR

Oral evidence at hearing regularly turns to extensive discussion over factors of DNA – TPPR, in a wide arc of how DNA can transfer from one item to another, intermediaries, contact pressure, material types, shedder status, time periods, bi-directional transfer and aerosol DNA to name a few. For obvious reasons, advocates commonly probe the expert in an effort to uncover areas which provide greater support to their respective version. These discussions are inherently capable of misleading the jury if not managed within a clearly defined and scientifically valid strategy.

DNA – TPPR Fact – When the expert report relates to source DNA only, the factors of DNA – TPPR have not been scientifically evaluated.

At the highest level, a jury should be led by science to understand that;

“The principles of TPPR are scientifically understood, but it is not understood what impact those factors have had on the source results in this matter, and science cannot say what is more or less likely”.

An expert’s oral responses on DNA – TPPR should consistently inform the courtroom that no evaluation of the factors has been conducted, and that the factors are general and informative only. When an expert is led or permitted to stray deeply into the factors on the stand, a jury is increasingly at risk of mistaking the conversation as evidence of a true evaluation, rather than it’s truer description as hypothetical or theoretical ruminations.

The greatest risk of prejudice to the jury with DNA-TPPR occurs when the expert casually inserts inherently plausible generalisations which seemingly speak of the results, when in fact they do not. An expert should contain their responses to those of an expert, resisting any temptation under questioning, to apply the speculations of a layperson.

DNA – TPPR misleading court speculations

Some of the more frequent examples;

– More DNA is more likely direct transfer.

– Every touch reduces the amount of DNA.

-This small amount of DNA makes indirect transfer less likely.

-Multiple transfer steps are less likely than direct transfer.

-DNA in this location is more likely direct transfer.

-There is data to support this opinion.

-Over the last 20 years of experience, it is in my opinion more likely.

• Following such speculative expert responses, thorough questioning should probe;

a) what scientific evaluation was utilised to form such a conclusion, and;

b) what data has been utilised to conduct the evaluation, and;

c) what defence and prosecution hypothesis were relied upon, and;

d) what data is evidenced.

DNA – TPPR Research v Court

The critical distinction between research and the courtroom is at times lost by legal practitioners and scientists alike. There is a large volume of research data available, with in excess of 60 published papers on DNA-TPPR and some 100’s of papers relating to DNA transfer. The purpose of these papers is ultimately as a contribution towards forensic science as an evidentiary tool, however it does not at all mean that one can merely speak of, or call up those papers in support of an ad-hoc opinion or speculation on the stand.

As one example, the court regularly and invalidly hears casually that;

“more DNA is more likely direct transfer” further that “there is data to support this opinion”

It is true that there is data broadly associating larger amounts of DNA with direct transfer, however the expert has fallen into the trap of failing to identify the distinction between research and what is valid in court. Such statements may well be suitable when pondering a study design at the lab, however they are nothing like evidence of an evaluation of the published data, and consideration of it’s specific suitability and reliability under the exacting prosecution and defence hypothesis.

Legal practitioners may often not immediately identify the fundamental fallacy of such expert statements. To put an analogy, it is as flawed scientifically, as a lawyer making submissions merely that;

“The defendant is more likely guilty as there is data supporting frequency of guilt”

Notes for legal practitioners – 2024

It is fundamentally understood that DNA can be transferred from one item to another by numerous means and with multiple intermediaries. It is appropriate for the courts to hear that there are factors which can affect DNA transfer. Some of these factors may include the surface material, DNA quantity, DNA source, shedder status, time since initial deposition and transfer, background DNA, DNA parking, friction and contact pressure.

 Generally, it is most appropriate to give the jury enough information to appreciate that there are known factors, however it can be confusing and misleading to make efforts that seek to educate the jury exhaustively on all the factors.

Generally, I would advise that the factors reasonably associated with the defence and prosecution version are more necessary, appropriate and pertinent for the jury to hear.

When these factors have not been scientifically evaluated as to the results (Activity Level Reporting) and the DNA results are source only, the jury must understand consistently that DNA – TPPR is general information only and does not represent a scientific evaluation of any sort in the matter itself.