Expert evidence — specialised knowledge of child behaviour

[5-300] The operation of ss 79(2) and 108C Evidence Act 1995

Last reviewed: June 2023

Section 79(2) Evidence Act 1995 provides that “specialised knowledge” based on a person’s “training, study or experience” in s 79(1) extends to “specialised knowledge of child development and behaviour (including specialised knowledge of the impact of sexual abuse on children and their development and behaviour during and following the abuse”: s 79(2)(a). The opinion of such a person includes an opinion relating to the development and behaviour of children generally and/or the development and behaviour of children who have been victims of sexual offences, or offences similar to sexual offences: s 79(2)(b).

Section 108C(1) also permits a party to call evidence from an expert but when it is relevant to the credibility of their own, or the other party’s, witness. Section 108C(2) is in identical terms to s 79(2).

Although the concept of “study” has limits, this limb of ss 79(1) and 108C(1) expressly contemplates a person giving evidence of an opinion that is wholly or substantially based on specialised knowledge based on “study” which necessarily involves scrutinising the work of others: AJ v R [2022] NSWCCA 136 at [73].

Leave is required for evidence under s 108C but not under s 79(2).

[5-310] Notes

Last reviewed: June 2023

In child sexual assault cases it is likely the Crown may seek to rely upon expert evidence regarding children’s behaviour. This will be indicated on the Crown Readiness Hearing Case Management Form filed with the District Court. If the defence proposes to rely upon such evidence this will be indicated on the corresponding defence Case Management Form. In such cases, it is prudent to raise this with the parties at the earliest opportunity to ensure any questions related to expertise, relevance and admissibility are dealt with before the trial commences.


Where reliance is placed on lengthy expert reports, and a ruling on admissibility is sought, the trial judge should require the adducing party to identify the parts of the report it seeks to adduce in oral evidence. A determination can then be made as to the facts in issue with respect to which the evidence is tendered: Aziz (a pseudonym) v R [2022] NSWCCA 76 at [94].


Evidence of this kind may not be required if the parties reach agreement as to the trial judge informing the jury of uncontroversial propositions, such as that “victims of child sexual abuse may respond [in ways] contrary to how we might expect victims to respond”. General evidence of this kind in an expert report is not calibrated to the issues in the trial, whereas a direction by a trial judge can be: AJ v R at [69]; see also Fagan J at [165]–[168].


Expert evidence generally will be admissible where it addresses the relationship between a child and a perpetrator (such as “familial situations” compared to non-familial or stranger perpetrators) and the effect on the child’s behaviour during and after the abuse. Such evidence is of a type approved in AJ v R [2022] NSWCCA 136 at [64]–[66]: BQ v R [2023] NSWCCA 34 at [233]–[239].


Section 108C [Evidence Act 2008 (Vic)] was considered in MA v R (2013) 40 VR 564. The provision is materially similar to the NSW provision. The court held that general opinion evidence concerning how a child may react to sexual abuse was admissible. However, it would be a rare case that an expert should be invited to express an opinion as to the actual behaviour of the alleged victim: MA v R at [100].

In Aziz (a pseudonym) v R [2022] NSWCCA 76 expert evidence regarding the behaviour of child sexual abuse victims was found to be opinion evidence and admissible under s 108C even though, unlike in MA v R, the expert did not express an opinion about the particular complainant’s credibility. The evidence was relevant as it was capable of assisting the jury in making its own assessment of the truthfulness of the complainant’s account: [92]. In the circumstances of that case, where the evidence was admitted without objection, the expert’s evidence was “opinion evidence” because it drew conclusions based on the published research of others in that particular field and was not simply a “literature review”: [77], [80]. However, an expert cannot express an opinion about a topic merely because they have read and reviewed the work of others. They must have their own qualifications (that is, training, study or experience) before they can do so: AJ v R at [74]–[75], [77]; see also BI (Contracting) Pty Ltd v University of Adelaide [2008] NSWCA 210 at [23].


In Clegg v R [2017] NSWCCA 125 at [122], it was held the judge correctly directed the jury that evidence admitted under s 108C could not be used to decide the truth of charges. The content of a direction for evidence adduced under s 108C will depend on the nature of the opinion evidence led by the Crown. The direction at [2-1130] Suggested direction — expert witnesses should be adapted accordingly.


The jury does not always need to be instructed the expert evidence says nothing about the credibility of the particular complainant. The need for such a direction will depend on the way the evidence is led, whether it was challenged in cross-examination, the approach taken by defence counsel at trial, and how the Crown uses the evidence in closing address: BQ v R at [268]–[269].