When an expert witness is tendered before the court, the attorney presenting the witness must advise the judge as to the specific field(s) of expertise in which the attorney wants the witness qualified.  At the conclusion of the qualification voir dire, in which the proposed expert’s qualifications are examined, and after hearing legal submissions, the judge will decide if the witness is sufficiently qualified to give expert evidence and in what specific field(s).  The decision of the judge sets the parameters of the subject matter on which the expert may opine.

Experts are only qualified to give opinion evidence in the permitted field(s) of expertise.  It is not proper, and in fact it is usually detrimental, for the expert to give evidence in areas outside of the prescribed field(s) of expertise, even if the expert knows the answer and if an attorney wants the expert to go down that road.  The expert must resist the temptation and the pressure to stray.

The dangers of transcending the stated boundaries are numerous:

First, given that the expert was not likely planning to transcend any boundaries, and therefore was not prepared to testify in a different field of expertise, the risk of the expert making an error in the non-qualified field is palpable.  Making an error in a line of questioning that should have never been acceded to could impact the overall evidence of the expert and the case itself.

Second, errors made by the expert in the non-qualified field can impact the credibility of the expert in the eyes of the trier of fact in respect of the evidence given in the qualified field.  This is an unnecessary sacrifice that must be avoided.

Third, once the expert opens the door to answering questions in other areas of expertise, it is very hard for the expert and the attorney to close it.

Finally, an unnecessarily helpful expert may come across as being too helpful, which may give rise to an imputation of bias by opposing counsel.  An expert who gets a reputation for straying will make attorneys who might otherwise call the expert to testify hesitant to do so.

An expert must always be cognizant of the limits imposed by the court’s stated qualifications.  Those limits set the boundaries within which the expert is obliged to remain.  Any attempts by attorneys to go beyond those limits must be resisted by the expert in a clear and professional manner. The expert should not wait for the attorney who is presenting the expert’s evidence to come to the rescue. In fact, it may be that attorney who is causing the problem!  The expert has total control over what he/she says in court.  No one can make an expert say what he/she does not want to say.  An expert is well advised to understand the proper parameters of his/her stated area of expertise and remain within them. In other words, stay in your lane!