Criminal Defenses: Insanity, Intoxication, Self-Defense, Duress, Necessity, Mistake

Insanity is an affirmative defense, and jurisdictions apply one of four tests. M'Naghten (the majority and California rule): the defendant, due to a mental disease or defect, either did not know the nature and quality of the act or did not know that the act was wrong. Irresistible impulse: the defendant lacked the capacity to control his conduct or conform it to the law, even if he knew the act was wrong. Durham/product: the crime was the product of mental illness (now nearly extinct).

MPC/ALI: the defendant lacked substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the law — a hybrid of M'Naghten and irresistible impulse. Distinguish insanity from competency to stand trial (a present-time inquiry into ability to understand the proceedings and assist counsel). Intoxication is a separate and heavily tested doctrine. Voluntary intoxication is a defense only to specific-intent crimes, and only insofar as it actually prevented formation of the required specific intent; it never excuses general-intent, malice, recklessness, or strict-liability crimes.

Involuntary intoxication (unknowing ingestion, coercion, or unexpected reaction to prescribed medication) is treated like insanity and may be a defense to any crime, including general-intent offenses, if it produces a qualifying mental state.

A person may use non-deadly force when he reasonably believes such force is necessary to protect himself from the imminent use of unlawful force. Deadly force may be used only when the defender reasonably believes he faces an imminent threat of death or serious bodily harm. The original aggressor generally cannot claim self-defense unless he withdraws and communicates that withdrawal, or the victim suddenly escalates a minor fight into deadly force.

The majority rule imposes no duty to retreat before using deadly force; a minority requires retreat before deadly force if it can be done safely, but even those jurisdictions recognize a 'Castle doctrine' exception in one's own home. California imposes NO duty to retreat — a person may stand his ground and even pursue an assailant if reasonably necessary — and has a strong home-defense presumption. Critically, California recognizes imperfect self-defense: an honest but unreasonable belief in the need for self-defense does not justify the killing but reduces murder to voluntary manslaughter (negating malice).

Defense of others permits force to protect a third person under the same reasonableness standard; the modern majority lets the defender act on his own reasonable appearance of the situation even if the person aided had no actual right to defend.

Duress is an excuse: the defendant committed the crime because of another person's threat of imminent death or serious bodily harm to himself or a family member (or, broadly, another person), and a reasonable person would have yielded. Duress is NOT a defense to intentional murder — one may not kill an innocent person to save oneself.

Necessity (the choice-of-evils justification) applies where natural forces (not human threats) create a situation in which the defendant reasonably believes that committing the crime is necessary to avoid a greater imminent harm; the harm avoided must outweigh the harm caused, and like duress it is unavailable to justify taking an innocent life, and it fails if the defendant created the emergency.

Mistake of fact negates mens rea and tracks the specific/general-intent framework: for specific-intent crimes, any honest mistake (even unreasonable) is a defense if it negates the specific intent; for general-intent and malice crimes, only a reasonable mistake works; for strict-liability crimes, no mistake of fact is a defense. Mistake or ignorance of the law is almost never a defense, with narrow exceptions: reliance on an official (later-erroneous) interpretation of the law, a statute not reasonably made available, or where knowledge of the law is itself an element of the offense.

Entrapment (a defense in the minority subjective form where the defendant lacked predisposition and the government induced the crime) and infancy round out the commonly tested defenses.

Mastering defenses requires attention to who bears the burden and to California's particular formulations. Insanity is an affirmative defense the defendant must prove, and California requires the defendant to establish legal insanity by a preponderance of the evidence under its M'Naghten test (Penal Code section 25), which California courts read in the conjunctive — the defendant must show he was incapable of knowing or understanding the nature and quality of his act OR of distinguishing right from wrong.

California abolished the diminished-capacity defense after the Dan White case (Penal Code sections 25 and 28), but it allows evidence of mental disease to negate the specific mental states of premeditation, deliberation, and express malice — 'diminished actuality' — which can reduce first-degree to second-degree murder or to manslaughter. This is distinct from insanity and is a frequent California essay trap. Self-defense burdens also matter: once a defendant produces evidence of self-defense, the prosecution must disprove it beyond a reasonable doubt.

California's stand-your-ground principle is delivered through jury instruction (CALCRIM 505/3470) rather than a single statute, and Penal Code section 198.5 creates a presumption of reasonable fear of death or great bodily injury when an intruder unlawfully and forcibly enters a residence — California's Castle-doctrine codification. Defense of property permits only non-deadly force to protect property; deadly force is never justified solely to defend property, though a defender may use deadly force if the intrusion also threatens the occupants (the felony or habitation context).

The defense-of-habitation rules can overlap with self-defense when an intruder enters a home. Defense of others tracks self-defense and, under the modern majority and California, lets the rescuer act on a reasonable appearance even if the person aided turns out to have had no right to defend.

Necessity in California is a judicially recognized justification requiring that the defendant acted to prevent a significant and imminent evil, had no reasonable legal alternative, the harm caused was less than the harm avoided, the defendant entertained a good-faith and objectively reasonable belief, and did not substantially contribute to creating the emergency.

Entrapment is the area of sharpest California divergence: while the MBE tests the federal subjective test (focusing on the defendant's predisposition), California uses an OBJECTIVE test (People v. Barraza) that asks whether the police conduct was likely to induce a normally law-abiding person to commit the crime — under California's test the defendant's predisposition is irrelevant, so a predisposed defendant can still win the defense if the police tactics were impermissibly overbearing.

Infancy at common law conclusively presumed children under seven incapable of crime, rebuttably presumed incapacity for ages seven to fourteen, and treated those fourteen and over as adults; modern juvenile-court systems largely supplant these rules. Finally, distinguish failure-of-proof defenses (mistake, intoxication negating intent), which simply negate an element the prosecution must prove, from true affirmative defenses (insanity, duress, necessity, self-defense), which the defendant must raise and sometimes prove — a framing point that organizes any defenses essay.

Defenses: Trigger and Scope