Why This Matters
Criminal defenses are the backbone of how our legal system balances accountability with fairness. You're being tested on more than just definitions—examiners want you to understand why certain defenses exist and when they apply. The key concepts here include mens rea (guilty mind), actus reus (guilty act), justification versus excuse, and constitutional protections. Each defense either negates an element of the crime, provides legal justification for the act, or invokes procedural protections that limit prosecution.
Think of defenses as falling into distinct categories based on what they challenge: some attack the mental element of the crime, others argue the act was legally justified, and still others invoke constitutional safeguards. When you encounter these on an exam, don't just memorize the names—know what element of criminal liability each defense targets and whether it leads to complete acquittal, reduced charges, or dismissal on procedural grounds.
Justification Defenses: The Act Was Legally Permissible
These defenses don't deny the act occurred—they argue it was the right thing to do under the circumstances. Justification defenses focus on the objective reasonableness of the defendant's conduct, essentially saying society approves of the choice made.
Self-Defense
- Requires reasonable belief of imminent harm—the threat must be immediate, not speculative or future-oriented
- Proportionality is essential: the force used must match the level of threat faced, meaning deadly force requires a deadly threat
- Duty to retreat varies by jurisdiction—some states require attempting escape before using force, while "stand your ground" states do not
Necessity
- Justifies illegal conduct to prevent a greater harm—the classic example is breaking into a cabin to survive a blizzard
- No reasonable legal alternative must have existed at the time; if you could have called 911, necessity fails
- Balancing test applies: courts weigh the harm caused against the harm avoided, and the avoided harm must clearly outweigh
Justification (General)
- Focuses on the rightness of the act rather than just preventing harm—overlaps with but is broader than necessity
- Commonly applies to defense of others, where you intervene to protect a third party from harm
- Objective standard: the act must appear necessary and appropriate to a reasonable person in that situation
Compare: Self-Defense vs. Necessity—both justify otherwise illegal acts, but self-defense responds to human threats while necessity typically involves natural circumstances or emergencies. On an FRQ asking about justification defenses, distinguish based on the source of the danger.
Excuse Defenses: The Act Was Wrong, But the Defendant Isn't Fully Culpable
Excuse defenses concede the act was harmful but argue the defendant shouldn't bear full responsibility due to circumstances affecting their mental state or free will. These defenses attack mens rea—the "guilty mind" required for criminal liability.
Insanity Defense
- Defendant couldn't understand the nature or wrongfulness of their actions due to severe mental illness at the time of the crime
- M'Naghten Rule is the most common standard: did the defendant know what they were doing, and did they know it was wrong?
- Verdict of NGRI (not guilty by reason of insanity) typically results in commitment to a mental institution, not freedom
Diminished Capacity
- Partial defense that acknowledges reduced mental ability without meeting the full insanity threshold
- Reduces charges rather than acquitting—first-degree murder might become manslaughter if the defendant couldn't form specific intent
- Requires psychological evaluation to establish the defendant's mental state at the time of the offense
Duress
- Defendant committed the crime under immediate threat of death or serious harm—someone held a gun to their head
- Threat must be imminent and inescapable; a threat to harm you "someday" doesn't qualify
- Cannot be used for murder in most jurisdictions—the law won't excuse taking an innocent life to save your own
Automatism
- Actions taken without conscious control, often due to sleepwalking, seizures, or other involuntary physical states
- Negates actus reus because there was no voluntary act—the body moved but the mind wasn't directing it
- Medical evidence required to establish the defendant genuinely lacked awareness or control
Compare: Insanity vs. Diminished Capacity—insanity is a complete defense leading to NGRI, while diminished capacity only reduces the severity of charges. If an exam asks about mental state defenses, know that insanity focuses on understanding right from wrong, while diminished capacity focuses on ability to form specific intent.
Defenses Negating Mens Rea: No Criminal Intent
These defenses argue the defendant lacked the mental state required for the crime. Without mens rea, most crimes cannot be established—these defenses target that essential element.
Intoxication
- Voluntary intoxication can negate specific intent crimes (like burglary or first-degree murder) but not general intent crimes
- Involuntary intoxication—being drugged without knowledge—can serve as a complete defense if it prevented forming any criminal intent
- Key distinction: getting drunk by choice limits your defense options; being drugged against your will expands them
Mistake of Fact
- Honest and reasonable false belief about a circumstance that negates criminal intent—thinking you're taking your own umbrella, not someone else's
- Must negate the specific mens rea required; if you'd still be guilty even knowing the true facts, the defense fails
- Reasonableness matters: an unreasonable mistake typically won't excuse conduct, though standards vary by jurisdiction
Mistake of Law
- Generally not a valid defense—"I didn't know it was illegal" almost never works because everyone is presumed to know the law
- Limited exceptions exist: reliance on an official but incorrect statement of the law, or when the statute requires knowledge of illegality
- Policy rationale: allowing this defense would incentivize ignorance and make prosecution nearly impossible
Compare: Mistake of Fact vs. Mistake of Law—mistake of fact can be a complete defense if it negates intent, while mistake of law almost never works. Remember: not knowing the facts can excuse you; not knowing the law cannot.
Procedural and Constitutional Defenses: Limits on Prosecution
These defenses don't address guilt or innocence—they argue the government cannot or should not prosecute due to legal constraints. These protections exist to check government power and ensure fairness in the justice system.
Double Jeopardy
- Fifth Amendment protection against being tried twice for the same offense after acquittal or conviction
- Same sovereign rule: federal and state governments are separate sovereigns, so both can prosecute for the same act
- Attaches when jeopardy begins—typically when the jury is sworn or the first witness testifies in a bench trial
Statute of Limitations
- Time limit for initiating prosecution—once it expires, the defendant cannot be charged regardless of guilt
- Varies dramatically by crime: minor offenses may have 1-2 year limits, while murder typically has no limitation
- Policy balance: encourages timely prosecution while recognizing that evidence degrades and witnesses forget over time
Entrapment
- Government induced the defendant to commit a crime they weren't predisposed to commit—it's about manufacturing criminals, not catching them
- Predisposition is key: if the defendant was already inclined to commit the crime, entrapment fails even if police provided the opportunity
- Subjective vs. objective tests vary by jurisdiction—some focus on defendant's predisposition, others on government conduct
Compare: Double Jeopardy vs. Statute of Limitations—both prevent prosecution, but double jeopardy applies after a trial has occurred while statute of limitations applies before charges are ever filed. Both reflect the principle that prosecution power must have limits.
Consent and Victim-Based Defenses
These defenses argue the alleged victim agreed to the conduct or the defendant simply wasn't present. They challenge either the harm element or the defendant's connection to the crime.
Consent
- Victim's voluntary agreement to the act can negate certain crimes, particularly in assault or sexual offense contexts
- Must be informed, voluntary, and competent—consent obtained through fraud, coercion, or from someone incapable doesn't count
- Limited application: consent cannot justify serious bodily harm or death in most jurisdictions, regardless of the victim's wishes
Alibi
- Defendant was elsewhere when the crime occurred—a factual defense that challenges identification, not legal elements
- Requires corroborating evidence such as witnesses, surveillance footage, receipts, or electronic records
- Complete defense if proven: if the defendant physically couldn't have committed the crime, acquittal follows
Compare: Consent vs. Alibi—consent admits the defendant was involved but argues the conduct was permissible, while alibi denies involvement entirely. Consent is a legal argument; alibi is a factual one.
Quick Reference Table
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| Justification (act was permissible) | Self-Defense, Necessity, Justification |
| Excuse (reduced culpability) | Insanity, Diminished Capacity, Duress, Automatism |
| Negating Mens Rea | Intoxication, Mistake of Fact, Mistake of Law |
| Constitutional/Procedural | Double Jeopardy, Statute of Limitations, Entrapment |
| Victim-Based/Factual | Consent, Alibi |
| Complete Defenses | Self-Defense, Insanity, Alibi, Involuntary Intoxication |
| Partial Defenses | Diminished Capacity, Voluntary Intoxication |
| Defenses That Rarely Work | Mistake of Law, Duress for Murder |
Self-Check Questions
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Which two defenses both negate mens rea but differ in whether the defendant's condition was self-induced? How does this distinction affect their application?
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Compare and contrast the insanity defense and diminished capacity. What outcome does each produce, and what standard of mental impairment does each require?
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A defendant claims they broke into a pharmacy to steal insulin for their diabetic child during a blizzard when no other options existed. Which defense applies, and what elements must they prove?
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Why does the law treat mistake of fact and mistake of law so differently? Identify one scenario where each might apply and explain the likely outcome.
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If a defendant is acquitted of robbery in state court, can federal prosecutors charge them for the same incident? Which defense is implicated, and what exception applies?