The defense can often put expert witnesses to good use, as well. For example, an expert witness familiar with the particular cultural issues involved in a child abandonment case might help the jury understand why conscientious parents from another culture might leave their children unattended for an extended period of time. An expert familiar with neuro-psychology might testify for the defense about abnormal brain development in a case where insanity is an issue. An expert might also testify for the defense about the suggestibility of child witnesses in a child abuse case. The list goes on and on.
However, before a witness is qualified to testify in any criminal trial their qualifications and testimony must meet threshold requirements under the Rules of Evidence. The Rules which apply to the testimony of experts are as follows:
Opinion Testimony by a Lay Witness: If the witness is not testifying as an expert, the witness' testimony in the form of opinions or inferences is limited to those opinions or inferences which are rationally based on the perception of the witness and helpful to a clear understanding of the witness' testimony or the determination of a fact in issue in the case.
Testimony By Experts: If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion.
Bases of Opinion Testimony by Experts: The facts or data in the particular case upon which an expert bases an opinion or inference may be those perceived by, reviewed by, or made known to the expert at or before the hearing. If of a type reasonably relied upon by experts in the particular field in forming opinions or inferences upon the subject, the facts or data need not be admissible in evidence.
So, as you strategize with your criminal defense lawyer about defending charges against you, ask them how they might employ the use of experts to assist in your defense. Although experts will charge a fee for their services, the money is often well spent and a make-or-break investment in your future.