A landmark decision by the California Court of Appeal in May 2016 gives mental health experts a greater role in the defense of criminal charges. The Herrera decision is already being cited as a basis for granting new trials to defendants who claim that judges improperly limited the testimony of their expert witnesses.

Evidence at the Herrera trial

Richard Arce Herrera was convicted of first degree murder. A witness testified that she saw Herrera stop his car in the street and chase someone who fled from the vehicle. Another witness later saw Herrera kneeling over the victim on the ground next to a car. Herrera appeared to be performing CPR. That witness saw Herrera enter his car and drive away.

The victim had been stabbed 21 times. He had shallow stab wounds in his back and deeper wounds in his chest and abdomen. He died from blood loss.

When Herrera arrived at his home, he appeared to be in shock. He told his parents that he had killed someone in self-defense after being attacked and raped. He showed his parents an injury on his side. His parents called 911.

Over a period of three years, beginning at the age of eight, Herrera was molested repeatedly. He was raped at the age of 15 and again at the age of 19. He became suicidal and obtained counseling to help him cope with his stress.

The murder victim was a childhood friend of Herrera’s. Herrera met him at a bar. The victim later began to touch Herrera inappropriately. Herrera testified that the behavior triggered flashbacks and caused him to have a panic attack.

Herrera agreed to drive the victim home because the victim was intoxicated. The victim began to touch Herrera again, then struck Herrera when Herrera rejected his advances. Herrera had a steak knife in the car’s console. He testified that the victim grabbed the knife and lunged at him, grazing his abdomen. A prosecution witness testified that Herrera had a circular abrasion on his abdomen that was not consistent with a knife wound.

Herrera testified that he took the knife from the victim after a struggle. Herrera said he began reliving the past and responded by stabbing the victim. After the victim jumped from the car, Herrera chased him. When he caught up with the victim, Herrera stabbed him repeatedly. In his mind he was stabbing all the people who had molested or raped him.

Herrera came out of his rage and took the victim back to his car, intending to drive him to a hospital. However, he could not lift the victim into his car. He then tried to perform CPR. Realizing that the victim was dead, he drove to his parents’ home.

Expert Testimony

During his trial, Herrera called two mental health experts as witnesses. Herrera’s psychiatrist testified that Herrera had suffered for some time from post-traumatic stress disorder (PTSD). Her last session with Herrera occurred two weeks before the killing.

The other expert’s testimony was critical to the appeal. Dr. Nancy Kaser-Boyd is a clinical and forensic psychologist. She evaluated Herrera after the killing. She confirmed that he suffers from PTSD. She also testified that the perception of a threat can trigger a “fight or flight” response in a person with PTSD. Under those circumstances, a traumatized person might enter a “peritraumatic dissociative state.” People in that state feel emotionally detached from their actions, as if they were watching someone else. That testimony was consistent with Herrera’s testimony about how he felt during the killing.

The trial court precluded Dr. Kaser-Boyd from testifying about Herrera’s mental state at the time of the offense. In particular, she was not allowed to testify that, at the time of the killing, Herrera was suffering from PTSD, that he was in a peritraumatic dissociative state, or that he was psychiatrically impaired.

Herrera was convicted of first degree murder. He appealed, claiming that trial court violated his right to present evidence by restricting the testimony of his expert witness.

Mental Health Testimony and the Right to Present a Defence

Criminal defendants have a constitutional right to present relevant evidence that tends to establish their innocence. Herrera argued that the right to present Dr. Kaser-Boyd’s testimony trumped section 28 of the California Penal Code, which bars the admission of mental health evidence to show that a defendant did not have the capacity to form a mental state (such as intent, premeditation, or malice aforethought) that is required as an element of the charged offense.

Section 29 of the Penal Code states that the trier of fact (in this case, the jury) shall decide “whether the defendant had or did not have the required mental state.” California courts have held that these sections do not prevent a defendant from basing a defense on the absence of a required mental state, but preclude the defendant from introducing expert testimony to establish that defense. In other words, a defendant can testify “I didn’t intend to kill him” but cannot bolster that testimony with an expert opinion as to whether he actually formed the intent to kill.

The California rule is common. Similar rules have frequently been challenged as violating the right to present a defense, but those challenges have typically been unsuccessful. In Herrera’s case, the court of appeal did not hold that sections 28 or 29 are unconstitutional, but read them narrowly in order to maximize an expert witness’ latitude to testify about a defendant’s mental state at the moment an alleged crime was committed.

The Appellate Court’s Decision

The court decided that Dr. Kaser-Boyd should have been permitted to testify that Herrera was in a peritraumatic dissociative state and was suffering from PTSD at the time the offense was committed. While that testimony implies that Herrera did not form the intent to kill, the court concluded that the testimony was admissible, provided that Dr. Kaser-Boyd did not expressly opine that Herrera did not intend to kill the victim. Inferences about Herrera’s intent at the time of the killing were for the jury to draw, based on Dr. Kaser-Boyd’s opinions about Herrera’s mental status and on the other evidence.

If the jury had drawn the inference that Herrera’s peritraumatic dissociative state relieved him of responsibility for a premeditated killing, it might have found him guilty of a less serious offense, such as second-degree murder or voluntary manslaughter. It might also have concluded that Herrera acted in self-defense or in “imperfect self-defense,” if Herrera mistakenly believed that his life or security was imperiled. The court therefore reversed Herrera’s conviction and ordered that he be given a new trial with the benefit of full testimony from his expert witness.

Implications of the Herrera Decision

Appellate courts in many states (like many state legislatures) are skeptical of psychological evidence that supposedly “intrudes” upon the jury’s obligation to decide whether a defendant had the state of mind required for a crime. As brain science continues to advance, however, courts are likely to become more comfortable with expert testimony that explains how a defendant’s mental condition likely affected him at the time an alleged crime was committed.

The Herrera decision is already being cited in support of a motion for a new trial filed on behalf of a California woman who was convicted of murdering her husband. Her expert was precluded from testifying about how her husband’s constant abuse during a period of 29 years could have affected her perception of the threat that her husband posed to her on the night that she shot him. In that case and others, the Herrera decision may open the door for defendants to place greater reliance on expert testimony that helps juries understand their behavior.