The Credibility of Expert Witnesses

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Part 3.7 of the Evidence Act 1995 (Cth), which relates to assessments of the credibility of witnesses does not apply to expert witnesses. However, courts separately still grapple with the “credibility” of expert witnesses. We discuss some circumstances in which the issue of the credibility of an expert witness might come into question, and what lawyers need to consider when engaging an expert under those circumstances.

1. There is publicly available information concerning the expert’s personal life

In the context of section 79 of the Evidence Act, experts demonstrate their “credibility” through showing that they possess the appropriate training, studying, and experience and, in addition, that these foundations of their expertise are logically related to the subject of their expert opinions (Makita, Dasreef).

However, experts who have garnered a reputation, whether positive or negative, outside of the scope of their professional work, may understandably elicit some hesitation in lawyers engaging them for expert witness work. What may emerge in these circumstances is the age-old concern about the extent to which personal reputation will affect an overall sense of the expert’s character and therefore the apparent credibility of their opinions. Is it proper for courts to take issue with the integrity of evidence presented by an expert whose personal integrity has already been the subject of scrutiny?

The starting point for these considerations should always be the ability of the expert to fulfill her duties under legislation and to the courts. That is, lawyers should consider that unless the expert has been dishonest about their credentials and experience, or is unable to provide a report that is based on her expertise and relevant to the facts of the matter, courts will not exclude evidence from an expert based on facts about their personal lives.

There are, of course, notable exceptions to this rule. For example, where an expert already the subject of media attention will be presenting evidence before a jury, it is conceivable that, in very rare cases, the status or reputation of an individual giving evidence may prejudicially affect the jury. Whilst courts hold trials with an obligation to mitigate this bias by providing effective instructions to the jury, lawyers should be wary of very special cases in which the court may decide that the extent to which an expert will unfairly prejudice a jury is substantial enough to amount to a miscarriage of justice.

2. The expert is a party in other ongoing court proceedings

In general, the fact that the expert is involved in legal proceedings unrelated to the case in which they are providing evidence should not affect their credibility. As outlined in the above section, so long as experts can provide an opinion in conformity with s 79 of the Evidence Act, courts are unlikely to act on this information.

There are, however, some special circumstances that may complicate the capacity of experts to provide unbiased and reliable opinions:

  • Firstly, where the ongoing proceedings the expert is involved in directly concerns their professional integrity or competence. In those circumstances, lawyers should consider awaiting the outcome of the trial before hiring the expert.
  • Secondly, where the ongoing proceedings, because of its demands on the time and energy of the expert, are likely to affect the ability of the expert to complete reports in a timely manner. Although not an issue of credibility, lawyers ought to consider how the personal commitment involved in appearing as party in litigation will present difficult circumstances for any individual to take on additional work. An incomplete or poorly constructed report is also an unpersuasive one. 

3. The expert is clearly advocating for a party, rather than impartially presenting their expert opinions.

Courts have reference to a criterion of “credibility” when judging an expert’s performance during cross-examination. Whilst courts most frequently assess the credentials of the experts and the logic of their reasoning to decide on which expert reports they should rely on, “credibility” remains one of a few factors courts nonetheless consider when determining the relative merit of conflicting expert reports. The Commissioner’s formulation of this factor for consideration in Coles Supermarkets Australia Pty Ltd v Blackwood [2015] QIRC 011 has been cited in a few recent cases involving disagreement in expert witness opinions:

[The court should consider whether the] …  finding could be described as one based on the credibility of expert witnesses, having regard to such things as whether the witnesses display signs of partisanship in the witness box or lack of objectivity, and whether they make proper concessions to the viewpoint of the other side.” [120] (emphasis added)

In many other cases, the “objectivity” of the expert witness has also been called into question by the other side in relation to the frequency and detail of expert communications with instructing lawyers. A not uncommon argument against the integrity of an expert witness is that their collaboration with lawyers in understanding case theory and collating the facts of the matter amounts to a kind of collusion. The expert, in these circumstances, is accused of having been prejudicially influenced by instructing lawyers to advocate for a case outcome and therefore manipulating their report findings accordingly.

For a fuller discussion of the scope of lawyer-expert communications, read our blog, which discusses in more detail the current stance of courts on the permissible extent of expert briefings. In summary, courts accept that complex cases will require frequent communication between experts and lawyers for the purpose of collating documents and understanding the complexity of the matter and the case as a whole.

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