What Are Syndrome and Mental Defect Defenses in Criminal Cases?

How they have been argued and applied in federal and California courts

By Super Lawyers staff | Reviewed by Canaan Suitt, J.D. | Last updated on April 20, 2023

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In order to obtain a conviction, the prosecution must prove every element of a crime.

However, in some cases, a criminal defendant may still get an acquittal on a criminal charge even if they are technically guilty of every required element of the offense. This is done by raising a successful affirmative defense.

The syndrome defense and the insanity defense (also called “mental defect” defense) are two examples of affirmative defenses. In this article, you find an overview of how these legal defenses apply in criminal cases in California.

Understanding the Defenses: Syndrome Defense and Insanity Defense

To successfully raise an affirmative defense in a federal court or state court in California, you must have a strong understanding of the required elements. Here is a brief overview of the syndrome defense and the mental defect defense in California:

Syndrome Defense

The syndrome defense is also referred to as the battered women’s syndrome (BWS) defense or the battered person’s syndrome defense.

Battered women’s syndrome is a well-recognized mental health condition. The California Evidence Code § 1107 says, “Expert testimony is admissible by either the prosecution or the defense regarding intimate partner battering and its effects, including the nature and effect of physical, emotional, or mental abuse on the beliefs, perceptions, or behavior of victims of domestic violence.”

In the 1996 case People v. Humphrey, the Supreme Court of California considered when a jury may look at evidence of BWS when it is offered to support a claim of self-defense to a murder charge. Overturning a lower court’s decision, the Supreme Court ruled that juries may consider such testimony in deciding the existence and reasonableness of a defendant’s belief that killing was necessary for self-defense. So, a person who is the victim of domestic violence or spousal abuse may be able to raise this in a defense to certain criminal conduct.

Insanity Defense

To successfully raise an insanity defense (or “mental defect” defense) in California, a person charged with a crime must prove that it is more likely than not that they were legally insane at the time of the offense. California uses the M’Naghten test for legal insanity. Under this test, the defense applies if they did not understand the nature of the criminal act or they did not understand that they were doing something morally wrong.

In many cases, a defendant who raises an affirmative defense—syndrome defense, insanity defense, etc.—does so by entering a dual plea. If applicable, a California defense attorney could help you plead “not guilty,” and also “not guilty by reason of insanity.”

An Affirmative Defense Defeats or Mitigates Otherwise Unlawful Conduct

Rather than denying the allegations against the defendant, an affirmative defense gives some other reason why the defendant committed the alleged acts. In other words, with an affirmative defense, a defendant introduces evidence about their mental state that absolves them of culpability or criminal responsibility for the underlying offense.

In general, the burden of proof shifts when a defendant tries to raise an affirmative defense. For example, under the requirement of the law (California statute CALCRIM No. 3450), a defendant raising an affirmative defense of insanity has the burden of proving legal insanity at the time of the crime by preponderance of the evidence.

Whether you are pleading a syndrome defense, insanity defense, or any other type of affirmative defense, it is imperative that you present compelling and well-organized evidence to support your case. If you have any specific questions or concerns about syndrome/mental defect defenses, contact an experienced San Diego criminal defense lawyer for immediate help.

If you’d like to learn more about this area of the law, please see our criminal law overview.

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