In some cases we may benefit from having an expert advise the jury.  These cases typically involve issues which are complex and beyond the comprehension of the average juror.  For example, in an injury case it is typical for the plaintiff to put on a medical doctor who can opine upon the injuries the plaintiff suffered.  Most jurors don’t have a solid grasp on the complexities of the human anatomy and/or medicine and thus can benefit from having an expert explain these things and provide an opinion as to these things.

As you see in the example above, the doctor because of his education and training is in a unique position to help the jurors better understand the topic at issue in the case.  Trial court judges have significant discretion in determining what subjects are proper for expert testimony.  The general rule is that an expert is not allowed to testify unless the jurors will benefit from that expert testimony.  This means that if the subject matter at issue is somewhat commonsensical, meaning the jurors can decide on the issue based on their own life experiences, the expert typically will not be permitted to testify.  The reason for this being that the courts have concerns that jurors give significant weight to the opinions of experts.  If experts were allowed to opine on topics which don’t require expertise then such expert testimony and conclusions might invade the province of the jury.

Before your trial judge will allow your expert to testify as an expert witness you will need to qualify him or her as being such an expert.  The Federal Rules of evidence are fairly liberal in this regard.  An expert witness can be qualified by education and training.  However an expert can also be qualified by his experience in the field even if he or she lacks the formal education.  Remember however that there must be a link between the expert’s qualifications and the subject matter to be opined upon.  Trial judges generally deem expert testimony admissible, particularly since the other side of the case is always free to call their own expert in rebuttal.

Beyond simply being an expert the courts will also assess the reliability of any potential expert testimony.  There have been several landmark cases which define reliability in this context.  States vary as to which standard they chose to apply.  At a basic level the court must determine the admissibility of the evidence by evaluating the scientific method employed in reaching any conclusions as well as the application of that scientific method to the facts at hand.  Expect your qualification of such expert to be challenged by such things as the training of the proposed expert, the reliability of the expert’s methods and the application of those methods to the facts at hand.

An interesting point to note regarding expert testimony is that such experts are free to rely on evidence which would otherwise be inadmissible at trial.  Of course, it still must be shown that such evidence that would otherwise be inadmissible is the type of evidence that experts in the same field regularly rely on in forming conclusions.  Mind you that if the expert relies on this type of evidence the trial court is free to order that disclosure of any of this otherwise inadmissible evidence be disclosed.  The court has discretion as to whether any otherwise inadmissible data will be disclosed to the jury.  The court wants to avoid a situation where a party might use the expert simply to introduce otherwise inadmissible evidence.  This is why the trial court can prohibit such disclosure to avoid this situation or order such disclosure where the court determines that the value of that disclosure to the jury outweighs any prejudicial effect it may have on the other side’s case.

Expert witnesses differ from lay witnesses in that expert witnesses are allowed to give their conclusions or ultimate opinions to the jurors.  Remember, that lay witnesses are generally prohibited from doing this.  That being said, an expert witness is prohibited from speaking to the mental state of a defendant in a criminal case when that mental state is an element of the crime.  This would again, invade the province of the jury.  An expert does not have to offer the basis for his or her opinion.  However if inquired into on cross examination then the expert witness will have to disclose the basis for reaching his or her opinion.