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Expert Evidence Founded on Speculation

Feb 2023

HG v R [1999] HCA 2


Expert Evidence Founded on Speculation


This was a criminal case involving sexual offences against a child. A psychologist named Mr M, gave expert evidence on behalf of appellant (defendant).

Medico-legal expert witness - doctor with stethoscope. Expert Opinion

Mr M had experience in counselling emotionally disturbed children, and in dealing with, and counselling, victims of child sex abuse. Mr M’s report and evidence was that although the complainant had been sexually assaulted, the perpetrator of the assaults had been her natural father and not the appellant. He suggested the complainant had “buried” the assault in response to the trauma of it and the trauma had been “resurrected” at a later date in circumstances which led her to blame the wrong person.

The natural father had in fact died when the complainant was four and a half years old, about 5 years before the complaints to police, and there was no evidence that the natural father had committed these acts, save for the theory of the expert.

The theory that the complainant had, in truth, been a victim of sexual abuse, but that the abuser was her natural father (since deceased) would have been important to the defence case, if there could be found an evidentiary basis for such a theory.



This hypothesis was not shown to have been based, either wholly or substantially, on Mr M’s specialised knowledge as a psychologist.  Mr M’s opinion was based on “a combination of speculation, inference, personal and second-hand views as to the credibility of the complainant, and a process of reasoning which went well beyond the field of expertise of a psychologist1

Mr M approached the complainant’s story with scepticism.  He did not put the contention that the abuse was committed by the natural father to the complainant, nor did he investigate the possibility of abuse by a third party.

Various competing logical possibilities existed as to how the abuse was perpetrated.  It was not demonstrated by Mr M, and it is unlikely, that it is within the field of expertise of a psychologist to form and express an opinion as to which of those alternatives was to be preferred.

The evidence the defence sought to lead from Mr M really amounted to putting from the witness box the inferences an hypotheses on which the defence case wished to rely.

Further, expert evidence should be confined to opinions which are wholly or substantially based on their specialised knowledge. “Experts who venture “opinions”, (sometimes merely their own inference of fact), outside their field of specialised knowledge may invest those opinions with a spurious appearance of authority, and legitimate processes of fact-finding may be subverted. The opinions which Mr [M] was to be invited to express appear to provide a good example of the mischief which is to be avoided.2

The appeal was dismissed.



This case re-iterates a common theme found in these case reviews – the need for the expert witnesses to stick to their field of expertise.  It was not in the expert’s remit to diverge from his role as an assessing psychologist to venture speculative theories about hypothetical occurrences.

Related to this is the need for all expert opinions to be founded on facts, or such assumptions as reasonable and fair, and which withstand scrutiny by a court.  If these facts or assumptions are successfully challenged, then this may in turn materially affect the opinion and the expert evidence provided.

Here, though, the expert went one step further and sought to become a mouthpiece for his client – by proposing an opinion based on speculation and unproven and unfounded facts it severely impacted the expert’s credibility, and also the basis of the defence/appeal.  Experts must be balanced and objective in their opinion, and to fail in this will have substantial ramifications for the underlying case.


To see how we can potentially assist in the training of expert witnesses, including training on expert witness duties, please contact us.

1 At para 41.
2 At para 44.

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