Expert Witness Opinion Testimony on Defendants’ Mental State


On the subject of defendant’s mental state, courts have opened the door to an extensive use of expert opinion testimony evidence, taking in consideration two relevant Federal Rules of Evidence, such as 702 and 704(b), and 103(a)(2) and 403 as well.

Rule 702 governs the admissibility of experts’ opinion testimony concerning scientific, technical, or other specialized knowledge and also provides that the witness must qualify as an expert and that there must be a justification for his testimony in that particular case in which she is testifying. Once Rule 702 test is met; then, Rule 704(b) comes into place, limiting the expert’s opinion evidence by prohibiting the expert from testifying as to whether the defendant had the mental state or condition that represents an element of the crime charged; in other words, rule 704(b) is a limitation on the admissibility of expert testimony on defendant’s mens rea.

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rule 704(b) is a limitation on the admissibility of expert testimony on defendant's mens rea.

Expert Opinion Testimony on Defendants’ Mental State

Rule 704(b) application extends not only to psychiatrists or other mental health experts, but also to all expert witnesses who are hired to render an opinion as to a particular defendant’s mental state or condition. Regarding Rule 403, expert opinion testimony on defendant’s mental state must also beat scrutiny under this rule; otherwise, it may be excluded if its probative value is substantially outweighed by the danger of unfair prejudice.

In the case United States v. Brodie where a husband and wife were charged with failure to file tax returns. The defense called an accountant to sustain the claim that their failure to file tax returns was not willful, and based upon Rule 704(b), the district court excluded the expert testimony, and the Ninth Circuit sustained the exclusion, stating that the accountant’s testimony was an opinion as to the Taxpayers’ willfulness, a mental state which is an element of the crime charged. Thus, this case clearly exemplifies Rule 704(b) prohibition where the court held that such a rule precluded expert testimony not only on whether the defendant had the mental state that encompass an element of the crime charged, but also on stated matters from which the jury might infer whether the defendant possessed the necessary mental state.

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Afterward, the court overturned the traditional interpretation of Rule 704(b), demonstrating that there was a difference between testimony in which an expert expressed an opinion as to the defendant’s mental state and testimony from which the jury could, but was not compelled, to infer that defendant did not know she was committing a crime. In the case United States v. Rahm, 993 F.2d 1405 (9th Cir. 1993), the defendant was charged with attempting to knowingly pass counterfeit currency. The defense offered expert testimony regarding defendant’s poor visual perception and that defendant consistently unnoticed significant visual details. This testimony had the tendency to strengthen the defendant’s claim that she did not know the money was counterfeit. At the end, the district court excluded the expert testimony, and the higher court reversed the conviction on the ground that such opinion testimony was not barred under Rule 704(b) because the district court improperly excluded the expert testimony evidence since the jury could have inferred from the expert’s opinion that the defendant did not have the requisite mens rea, but that such an inference was not compelled.

Also, it is worth mentioning that as a general rule under the FRE 704(a), experts may testify as to their opinions on ultimate issues to be decided by the jury since some courts have found that the ultimate issue rule is unduly restrictive, difficult of application that only deprive the jury of useful information. In the case People v. Valdez (1997) 58 Cal.App.4th 494, 506.), Expert testimony may be premised on reliable material that is not admitted or even inadmissible at trial if it is the type of material upon which experts in that field reasonably rely in forming their opinions.

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Later on, in Morales, the defense attorney was representing a bookkeeper charged with making false entries in a union ledger. Then, the defense attempted to introduce expert testimony from an accountant regarding Morales’s knowledge and understanding of bookkeeping principles in which such evidence was offered to sustain Morales’s testimony that she had never intended to make false entries and that she only did not know that the entries she made were incorrect. The district court excluded the expert testimony, stating that it was a conclusion that was within the province of the jury. Subsequently, it was reversed on the ground that the expert testimony was not barred by Rule 704(b) since the accountant would not state an opinion that Morales did not intend to make false entries. Instead, the expert opinion was offered to state Morales weak grasp of bookkeeping knowledge. 

Finally, in Morales, the court construed Rule 704(b) prohibition more narrowly regarding expert testimony on defendant’s state of mind and explicitly overruled Brodie, the leading case in this matter, stating that Rule 704(b) excluded only testimony as to a defendant’s actual mental state during the charged offense or testimony that necessarily would imply that ultimate conclusion. Thus, Morales created an opportunity for defense attorneys and prosecutors to use creatively a large variety of expert testimony to illustrate complex issues to the jury related to the defendant’s mental state.

Related Stories:

Adversarial Bias: Expert Testimony in Modern Litigation

Expert’s Principles & Methodology “Key” to Admissibility

The Acceptable “Rate of Error” in Expert Witnesses’ Opinion