DNA Wrongful Conviction in 2024

In 2014, the High Court quashed the Fitzgerald conviction which erroneously relied upon the DNA evidence to place the defendant at the crime. The scientific theory and principles remain largely unchanged, making a repeat DNA wrongful conviction on the same evidence possible in 2024.

A deeper expert analysis of the DNA evidence in the High Court judgment can be read here: DNA wrongful conviction.

The murder conviction and resultant life sentence turned on the question as to whether the defendant directly deposited his DNA onto the didgeridoo within the premises, or whether it was transferred innocently by Grant Sumner. Crucially, the Crown relied upon DNA expert witness testimony whilst the defence did not.

The prosecution expert did correctly acknowledge the scientific fact that Daniel Fitzgerald’s DNA could have been transferred into the scene by Mr Sumner, whilst she did also proffer oral opinion that ran contrary to this fact.

Dr Henry opined support for the prosecution hypotheses variously as “most likely”, “more likely, and “much more likely”.

“Expert witnesses, however skilled or eminent, can give no more than evidence. They cannot usurp the functions of the jury or Judge sitting as a jury, any more than a technical assessor can substitute his advice for the judgment of the Court.” 1 (Makita (Australia) Pty Ltd v Sprowles)

The Appeal Court was apparently influenced by the expert oral probabilities when unanimously rejecting the Fitzgerald appeal;

“in light of Dr Henry’s evidence, the alternative hypothesis of a secondary transfer of the appellant’s DNA to the didgeridoo by Sumner was “extremely unlikely”’. 2 (Fitzgerald v The Queen [2014] HCA 28 (13 August 2014))

The Fitzgerald High Court judgement did not point to any evidence, data, or explanation by the expert witness as to the basis or theory that was relied upon in the formulation of the likelihoods.

“an expert witness must “explain the basis of theory or experience” upon which the conclusions stated are supposed to rest, for, as Sir Owen Dixon said in an extra-judicial address quoted by Fullagar J, “Courts cannot be expected to act upon opinions the basis of which is unexplained”. 3 (R v Jenkins; ex parte Morrison [1949] VicLawRp 51; [1949] VLR 277 at 303, Fullagar J).

DNA Likelihoods in Court

It is correctly acknowledged within the forensic sciences, that likelihoods in evidence are not arbitrary, subjective nor speculative. Likelihoods must be formulated and evidenced using a validated body of knowledge.

“Validation of scientific techniques is a key component in regard to the integrity and admissibility of scientific evidence within the courtrooms.” 4 (Dealing with DNA evidence in the courtroom, published in the Wrongful Conviction Law Review in 2020)

It simply is not the case that an expert witness can apply a likelihood that is based merely on personal opinion, even should that person happen to be an experienced, credible and reputable expert.

The International Society of Forensic Genetics (ISFG) has provided further guidance to practitioners stating that verbal scale cannot be used in isolation of a numeric likelihood ration.

“The verbal scale is optional but cannot be used by itself. If it is used, then the numeric equivalents must also be available/provided. In practise, one would provide first one’s likelihood ratio, then the verbal equivalent is applied afterwards”. 5 (Gill, P., et al., DNA commission ISFG. Evaluation of biological traces considering activity level propositions. Forensic Sci Int Genet, 2020.)

The European Network of Forensic Science Institutes (ENFSI) Guideline provides support to this rational, going further to remind practitioners as to their obligations before the courts. 6 (ENSFI Guideline for Evaluative Reporting in Forensic Science. 2015.)

“These probability assignments shall still be expressed by a number between 0 and 1 rather than by an undefined qualifier (such as frequent, rare, etc.). Such personal probability assignment is not arbitrary or speculative, but should be based on a body of knowledge that should be available for auditing and disclosure.”

In 2020, the matter was considered in some detail in the Wrongful Conviction Law review published paper titled Dealing with DNA evidence in the courtroom, in which Weathered et al formulated opinions as to the reliability of the evidentiary relationship around DNA transfer and the assignment of likelihoods.

“it is suggested that forensic biologists need to more clearly articulate the assumptions, limitations and sources of error associated with activity level DNA evidence – or alternatively, not provide an expert opinion of this form of evidence.”

Weathered et al, went further to say;

“this case [Fitzgerald] nevertheless highlights a question for criminal justice systems more broadly as to whether DNA transfer evidence is properly understood and evaluated, whether there is an appreciation of the limitations and potential error involved in this kind of evidence, and whether there is an awareness of the underlying assumptions used by scientists to rank the DNA transfer scenarios.”

Over the last 20 or so years in the courts, I have found prosecution oral likelihood opinions to be common, though less frequently tendered when I have been instructed, for obvious reasons.

Such unsupported likelihoods tend to be run at hearing, with them not having been tendered in written expert evidence. Meaning to some extent, defence is denied the opportunity to weigh and substantially challenge such evidence.

Juries can find themselves readily persuaded by the casual and subtle application of “more likely” or “extremely likely”, particularly when delivered by an unchallenged expert who may well appear to the jury as skilled, credible, and impartial.

Absent of a disclosed and auditable body of knowledge in support, and a numeric qualifier, such verbal likelihoods are scientifically invalid.

Helen Roebuck
Principal Forensic Scientist and DNA Expert Witness
Roebuck Forensic