Expert testimony may be used in criminal trials in Tennessee when it can substantially assist the trier of fact to understand the evidence or to determine a fact at issue. It is not uncommon for there to be evidentiary disputes about whether a particular expert's testimony on a particular subject does that. A trial court judge, guided by legal precedent, must ultimately determine whether particular expert testimony will be admissible. Then the trier of fact (a jury in most criminal trials) must determine how much weight to afford that evidence. In Tennessee criminal trials for first degree premeditated murder, the defense may present expert testimony regarding the defendant's mental state only if the expert testimony can conclude that the defendant 1) suffered from a specific mental disease or defect; and 2) was unable to form premeditation as a result of that mental disease or defect. If the proffered expert(s) on the defendant's mental state does not reach both these conclusions, the testimony will be inadmissible. Additionally, the expert may not testify on the question of whether he or she believes the defendant experienced "sufficient provocation" or acted in the "heat of passion," as those determinations are not scientific or medical conclusions.
In the recent case of State v. Thomas, W2013-02762-CCA-R3-CD (Tenn.Crim.App. 5-6-2015), the defendant, who was charged with first degree murder, sought to use a defense expert at trial to assist in rebutting the element of premeditation and to help establish that the defendant's criminal actions were undertaken in the "heat of passion." In a pre-trial hearing, the expert witness said the defendant was "overwhelmed" with his life and acted "explosively." However, the expert was unable to conclude that the defendant was incapable of premeditation. As long as a defendant is capable of premeditation, Tennessee courts consider the question of whether premeditation was actually formed to be no longer a medical diagnosis but simply a fact determination for a jury. Likewise, the question of whether a defendant acted with "sufficient provocation" or in the "heat of passion" is not a scientific or medical question for an expert but rather simply a judgment for the trier of fact to make.
For more information on when expert testimony may be admissible in criminal trial, contact Hindman & Associates.