Mental Health Defenses—Diminished Capacity and Voluntary Intoxication: What Can the Expert Say? 

In mounting a diminished capacity or voluntary intoxication defense in a first-degree murder case, the defense attempts to “negate” the element of specific intent, premeditation, or deliberation. To negate in this context means to raise a reasonable doubt about a mental element required for conviction. The diminished capacity defense involves negation through evidence of a mental disorder or emotional disturbance, whereas the voluntary intoxication defense involves negation through evidence of intoxication arising from the ingestion of alcohol or drugs. Though various types of evidence may be relevant to these defenses, expert testimony pertaining to the defendant’s mental state at the time of the alleged offense is often central. See State v. Shank, 322 N.C. 243, 248–49 (1988). Questions arise as to what an expert witness may say in court. Prior to 1983, it was generally not permissible under North Carolina law for an expert to testify on an ultimate issue to be determined by the jury. Rule 704, Official Commentary. However, Rule 704 now provides that “[t]estimony in the form of an opinion or inference is not objectionable because it embraces an ultimate issue to be decided by the trier of fact.” Experts are thus allowed to go further in offering testimony pertaining to a crucial fact that has a significant bearing on guilt or innocence. However, where the expert applies a particular legal standard to the facts at issue, such testimony likely goes too far and strays into impermissible “legal conclusion” territory. See State v. Parker, 354 N.C. 268, 289 [...]