Expert witness bias and its effects on the justice system
Expert witnesses provide their professional opinion about a relevant area they specialise in that relates to the case. Other witnesses in court are limited to relaying facts of what they saw or heard and are generally not allowed to express their opinion, unlike expert witnesses.
The rules governing the admission of expert witness opinions as evidence are covered in Part 3.3 of the Evidence Act 1995 (Cth) (Act), specifically section 79 which states that ‘if a person has specialised knowledge based on the person’s training, study or experience, the opinion rule does not apply to evidence of an opinion of that person that is wholly or substantially based on that knowledge.’ The ‘opinion rule’ contained within section 76(1) of the Act says that ‘evidence of an opinion is not admissible to prove the existence of a fact about the existence of which opinion was expressed.’ However section 79 which states that ‘if a person has specialised knowledge based on the person’s training, study or experience, the opinion rule does not apply to evidence of an opinion of that person that is wholly or substantially based on that knowledge.’ Expert evidence is usually only admissible if it complies with the Expert Witness Code of Conduct.
In a 2020 LawyersWeekly article, Dr Jason Chin, a leading legal scholar in this area, expressed concerns that expert witness partisanship and ‘junk’ science is ‘finding its way into courtrooms’ due to ‘unscrupulous experts willing to tailor their evidence to the needs of their instructing client.’ In his 2020 Sydney Law Review article, Chin identified that Australian courts have ‘resisted reading into a s 79 requirement that expert opinion be shown to be reliable’, meaning an expert’s bias – conscious or otherwise – towards the party paying their fees does not affect the admissibility of that evidence.
Still, evidence given by experts found to be blatantly partisan towards their instructors is given little weight by courts, such as in Universal Music Australia v Sharman Licence Holdings  FCA 1242 where his Honour Wilcox J at  reproached expert evidence given on one party’s behalf for being ‘little more than a partisan polemic.’
You can learn more about expert evidence from the expert’s perspective in Episode 10 of Hearsay with John-Henry Eversgerd, Senior Managing Director of Forensic Litigation Consulting at FTI Consulting.