BEING A WITNESS AND AN EXPERT
Ordinary witnesses may only testify to what they have directly experienced through their five senses. This testimony must be factual in nature and the witness, in nearly all cases, is barred from offering opinions, conjectures, or hypothetical information. Unlike other types of witnesses, however, expert witnesses are allowed to offer their opinions about evidence or situations that fall within their area of expertise. These opinions are allowed because the scientist is an expert in that area and knows more than anyone else in the courtroom about that topic; their opinion and expertise will assist the trier of fact in deciding the case. Scientific evidence can be powerful. It can also be suspect. Judges and juries may ignore an expert’s opinion evidence because it is just that: the expert’s view on that issue. Often, however, those opinions and views are based upon solid scientific data generated through valid analyzes and therefore have a firm basis, in fact. Expert witnesses must always remember that they are speaking for the evidence. They must be an advocate for the evidence and not for one of the parties in the case. When a forensic scientist testifies, he or she does so as an expert witness, that is, someone who knows more about a topic or subject than the average person. The scientist is brought to court by either the prosecution or defense and offered as an expert in some area of study that will aid the judge or jury (generically referred to as “the trier of fact”) in reaching their verdict. The scientist then undergoes a process of establishing his or her education, training, experience, and expertise in that discipline. The scientist will often need to cite his or her educational degrees, work history, previous testimony experience, publications, professional associations, and other relevant information that will justify their expertise to the court. The attorney offering the scientist as an expert asks questions that will lay a foundation for the scientist’s credentials; the opposing attorney may then ask questions in an attempt to weaken that foundation. This process is called voi dire, which is French for “speaking true.” It is important for the scientist to provide relevant qualifications to the court: being coach of the local high school soccer team has no bearing on whether someone should be considered an expert in drug analysis, for example. If the court rules that the scientist does possess sufficient credentials, then he or she may testify on that subject in the case at hand. The scientist must be careful to remain within the bounds of their expertise. It may be tempting to answer questions at the margin of your experience and offer speculative answers to be helpful or sound authoritative— but don’t do it! Few things will reduce your expertise in the jurors’ minds faster. The following fictitious example of over-extended testimony may clarify this idea.
Attorney: Dr Medical Examiner, what type of wound was found on the victim’s head? Dr ME: It was a contact gunshot wound. Attorney: Would the perpetrator have had gunshot residue on his or her hands? Dr ME: Undoubtedly. Gunshot residue would have been on their hands. Attorney: What is gunshot residue, Dr? Dr ME: Small particles of material expelled from the weapon when it is discharged. Attorney: What are those small particles composed of? What elements? Dr ME: Uhm … Attorney: What’s the best method to analyze gunshot residue? Dr ME: Scanning electron microscopy. We have one in our lab. Attorney: How does it work? Dr ME: Well …
The medical examiner obviously has overstepped his bounds of knowledge and is now in danger of looking foolish or arrogant to the jury. Although he knows something about gunshot residue, he is clearly not an expert in this area and should have answered the attorney’s second question with something like, “I’m a forensic pathologist and do not have specific expertise in the analysis of gunshot residue.” It is better to answer truthfully with “I don’t know” than to exceed your limits of knowledge, training, or experience.
