Some Daubert opponents often argue that any reliability test is improper for expert testimony, and eyewitnesses’ testimony is not subject to any special reliability test.
In this regard, the modern rules for expert testimony assert that such testimony is susceptible to “adversarial bias,” which refers to witness bias that occurs because experts are hired to persuade the claim of the hiring party in litigation; and thus, adversarial bias is presented by experts who will consciously comply their testimony with the trend of the attorney who hires them.
Although, a lay witness can also have conscious bias, it is not as serious as an experts’ opinion because lay witnesses are not paid for their testimony. This eradicates the chance of testifying as a paid witness since the scope of lay witness’s opinion testimony is limited to their own perceptions. Generally in any particular case there is a restricted group of possible ordinary fact witnesses; and the jury would be most likely to believe that an expert witness, especially a scientist, is an impartial witness in litigation.
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Although, for the most part is likely to impeach a lay witness’s testimony by showing the foundation of their bias; for example, their financial outcome in the litigation, or a relationship with a party, the opposing attorney will certainly find it very difficult to impeach a hired expert for being paid for his testimony since the opposing attorney may have his own expert witness who may be open and accurate on his payment for litigation, so even the conscious bias of a paid expert will probably not succeed on cross-examination.
Likewise, expert witnesses’ testimony in the vast categories of expertise is also susceptible to adversarial bias that may be affected by unconscious bias. This represents a constant problem with regard to expert witnesses’ testimony since there is always a tendency to be helpful to those who employed the experts and satisfactorily compensated them. Because expert witnesses’ success mostly depends on favorable evaluations from hiring attorneys or prosecutors, this can lead to reach conclusions favorable to the hiring party in litigation.
The selection bias is also another type of adversarial bias, meaning that the expert’s opinion will stand for the point of view that the hiring attorney wants to present at trial. Frequently, experts are selected if their opinion is known to support the attorney’s position. As a consequence, the court does not get fair expert opinion, from each party’s experts, but an incomparable opinion from each side.
Therefore, this is the attorneys’ reason to retain those experts whose views happen to coincide with the hiring attorneys’ theory of the case. Even if the attorney chooses not to employ an expert or cannot find one, selection bias will allow the attorney to find friendly experts. If so, the jury will receive a false perception of the matter at issue, especially when the expert opinion significantly favors one side. Thus, based on the particular circumstances, the jury may not hear from any expert whose views represent friendly expert opinions.