A defense expert witness is someone who is called upon during a jury trial to testify for the defendant. The witness typically has significant specialized training, skill, and education to be recognized as an expert on a particular topic by the court. Generally, the defense calls the expert witness to the stand to testify in hopes that the evidence the witness provides will sway the jury toward his or her side.
The jury trial process as practiced in many western countries involves a group of individuals who are sworn to deliver a fair verdict as to their belief in the events of a case, often to find a person or entity guilty or not guilty of a crime. Generally, the jury process evolved from medieval England. In fact, the Magna Carta addressed the importance of a trial by peers as early as 1215.
In a jury trial, both the prosecution and defense counsel attempt to sway the members of the jury with arguments, evidence, and testimony from witnesses. Lawyers understand that a wide number of factors are at play on the group dynamics that occur when juries deliberate and attempt to reach a consensus. These are hard to predict and to control. It is widely believed that the best way to sway a jury is through overwhelming evidence and information. Counsel for the defendant, therefore, will often hire a defense expert witness – whose testimony they hope will be seen as above refute – in an effort to convince the jury of the facts of the case.
A defense expert witness will be presented to the jury with his or her credentials so that jurors will have a sense of what qualifies this witness as an expert and to impress upon them the significance of his findings. The court allows the defense expert witness to present his findings so long as that his or her testimony is based on fact and reliable methods, and that he or she applied these methods appropriately to the information as it pertains to the case. Though he legally must share evidence he or she may have discovered both in support and against the defendant, counsel for the defense typically knows prior to hiring the expert what he or she is likely to say while on the stand based on his or her previous trial experience or published attitudes. At times this witness may be countered by the prosecution with its own expert witness, refuting the defense witness. In one of the shortcomings of the jury system, the jurors, most likely laymen in regards to the issue at hand, will have to determine which expert is most believable.
Overall, the scope of the defense expert witness is constantly evolving. The earliest known expert witness was a civil engineer and since that time, expert witnesses have been used to testify on things from mental health to economics. As technology becomes more prevalent in legal cases, defense expert witnesses are called on more and more.