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Court Holds Arizona Murder Defendant Cannot Introduce Evidence of Brain Damage

Last month, a state appellate court issued an opinion in an Arizona homicide case discussing whether the lower court properly excluded evidence of the defendant’s brain damage. The court acknowledged that a defendant is permitted to introduce evidence showing he had a character trait for acting impulsively. However, ultimately, the court concluded that he could not present evidence of brain damage to corroborate that trait. As a result, the court affirmed the defendant’s conviction for murder.

According to the court’s opinion, the defendant was charged with murder after he shot his then-girlfriend. Evidently, the defendant’s girlfriend ended their relationship. Two days later, she went to his home to return a gift. The defendant begged her to stay, and she refused, taking the couple’s children with her. The defendant allegedly followed his ex-girlfriend, blocked her car, and then shot her.

The state charged the defendant with first-degree premeditated murder. In support of his defense, the defendant sought to admit testimony from a medical doctor. The doctor planned to testify that the neuropsychological tests he performed were “consistent with significant and permanent diffuse brain damage.” According to the doctor, this meant the defendant was “more likely to have a character trait for impulsivity.” The prosecution objected, and the court precluded the admission of the doctor’s testimony. The defendant was convicted, and appealed the court’s decision to exclude his expert’s testimony.

The court began its analysis by noting that, aside from an insanity defense, Arizona law does not allow a defendant to introduce evidence of mental illness to negate the mens rea, or intent, element of an offense. However, the court went on to explain that evidence of a defendant’s behavioral tendencies is admissible solely to negate the intent element of a first-degree murder charge. The rationale being that evidence suggesting the defendant acted impulsively, rather than in a deliberate manner, was relevant to the intent element of first-degree murder.

Returning to this case, the court upheld previous decisions precluding the admission of evidence suggesting a defendant suffered from a mental illness that negated the intent requirement of the crime. The court drew a distinction between evidence of a character trait, such as impulsivity, and the underlying diagnosis that may have caused the presence of that trait, such as brain damage. Thus, the court concluded that “mental disease or defect evidence could not be admitted to show that a defendant was less likely to have formed the mens rea element of a crime even if that evidence corroborates behavioral-tendency evidence.”

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