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 (2001).

As a rule of thumb, where a phrase is complex and can be broken down into simpler terms, this may indicate that the phrase is an impermissible legal conclusion. Legal conclusions (based on the application of legal standards) are for the jury, not the expert, to make. Legal standards also may include terms of art. The expert, who likely has a background in medicine or psychology, may be in no better position than the jurors to apply these terms as they should be understood in the context of a given statute or as interpreted in the caselaw. See Rule 702(a) (expert testimony must “assist” the trier of fact); State v. Boyd, 343 N.C. 699, 708–10 (1996) (medical expert acknowledged lack of “precision” in his comprehension of the applicable legal standard). In contrast, where a phrase is relatively simple, this may indicate that the phrase is permissible testimony addressing an ultimate issue of fact. This rule of thumb may be somewhat counterintuitive given that the generally more sophisticated expert is prevented from using the more sophisticated language and the jury is left to wrestle with such language without the expert’s assistance. The jury will, however, have the benefit of the judge’s instructions in applying the legal standard at hand.

The distinction between permissible testimony on an ultimate issue and impermissible testimony involving a legal conclusion can be subtle and challenging to draw. The rule of thumb above is just that: it does not explain all the distinctions the courts have made in this context. Courts also consider other criteria such as whether the expert is qualified to use the terms properly, and whether the testimony uses the precise legal terms of art the jury must apply or whether it speaks to a dispositive question without explicitly establishing that an element has or has not been met.

The chart linked here gives concrete examples from North Carolina appellate courts, dividing common phrases into the categories of proper and improper testimony.