Types of Legal Defenses

Why This Matters

Understanding legal defenses is essential for grasping how criminal law balances accountability with fairness. You're being tested on more than just definitions—exams want you to recognize when a defense applies, why it negates criminal liability, and how different defenses operate through distinct legal mechanisms. The key concepts here include mens rea (criminal intent), justification versus excuse, and the relationship between individual culpability and external circumstances.

Don't just memorize a list of defense names. Know what element of a crime each defense attacks—whether it negates intent, challenges the act itself, or argues the defendant shouldn't be held responsible despite committing the act. When you can categorize defenses by their underlying logic, you'll handle any exam scenario thrown at you.

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Justification Defenses: The Act Was Right Under the Circumstances

These defenses argue that while the defendant committed the act, it was the correct thing to do given the situation. The law recognizes that sometimes breaking a rule prevents greater harm or protects important rights.

Self-Defense

• Reasonable belief of imminent harm—the defendant must genuinely and reasonably perceive an immediate threat, not a future or speculative one
• Proportional force is required; you cannot use deadly force against a non-deadly threat without losing the defense
• No duty to retreat exists in many jurisdictions (stand your ground laws), though some states require retreat if safely possible before using force

Necessity

• Lesser evil principle—the defendant chose to break the law to prevent a greater harm from occurring
• Imminent threat must exist with no legal alternative available; you can't claim necessity if calling 911 was an option
• Balancing test applies: courts weigh whether the harm avoided genuinely outweighed the harm caused by the criminal act

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Excuse Defenses: The Defendant Isn't Blameworthy

These defenses concede the act was wrong but argue the defendant shouldn't be held responsible due to circumstances affecting their mental state or free will. The focus shifts from the act to the actor.

Insanity Defense

• Cognitive incapacity—the defendant couldn't understand the nature of their actions or distinguish right from wrong due to mental illness
• M'Naghten Rule is the traditional standard (did they know what they were doing was wrong?), though some jurisdictions use broader tests
• Verdict of not guilty by reason of insanity typically results in commitment to a mental health facility, not release

Duress

• Coercion by another person—the defendant committed the crime because someone threatened them with serious, imminent harm
• No reasonable alternative must have existed; if escape or seeking help was possible, duress fails
• Murder exception applies in most jurisdictions—duress cannot excuse intentional killing, reflecting the law's hierarchy of protected interests

Intoxication

• Voluntary intoxication can negate specific intent crimes (like burglary) but not general intent crimes (like battery)
• Involuntary intoxication—being drugged without knowledge—can fully excuse conduct if it prevented forming any criminal intent
• Diminished capacity doctrine in some jurisdictions allows intoxication to reduce charges rather than eliminate liability entirely

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Defenses That Negate Criminal Intent

These defenses attack the mens rea element directly, arguing the defendant lacked the guilty mind required for conviction. No intent, no crime (for crimes requiring intent).

Mistake of Fact

• Honest and reasonable belief in incorrect facts can negate intent—like taking someone's identical umbrella believing it's yours
• Reasonableness standard applies; the mistake must be one an ordinary person might make under the circumstances
• Mistake of law is different—not knowing something is illegal is almost never a defense (ignorance of the law is no excuse)

Consent

• Victim's voluntary agreement to the conduct can negate crimes where lack of consent is an element (like theft or certain assaults)
• Informed and freely given consent is required; consent obtained through fraud or coercion is invalid
• Public policy limits prevent consent from excusing serious bodily harm or death in most jurisdictions—you cannot consent to being murdered

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Defenses Challenging Government Conduct

These defenses focus not on the defendant's actions but on how the government obtained evidence or initiated prosecution. They protect constitutional rights and ensure fair process.

Entrapment

• Government inducement occurs when law enforcement persuades someone to commit a crime they weren't predisposed to commit
• Predisposition test is key—if the defendant was already inclined to commit the crime, providing an opportunity isn't entrapment
• Subjective vs. objective standards vary by jurisdiction; some focus on defendant's predisposition, others on whether government conduct was improper regardless

Statute of Limitations

• Time bar on prosecution—if the government waits too long to charge someone, the case must be dismissed regardless of guilt
• Varies dramatically by crime: minor offenses may have 1-3 year limits, while murder typically has no limitation
• Policy rationale includes protecting defendants from stale evidence and encouraging prompt prosecution while memories and evidence are fresh

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Defenses Based on External Evidence

Alibi

• Physical impossibility—the defendant couldn't have committed the crime because they were elsewhere at the time
• Corroboration required through witnesses, surveillance footage, receipts, or other evidence placing the defendant at the alternate location
• Burden shifting occurs: once a credible alibi is presented, the prosecution must disprove it beyond a reasonable doubt

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Quick Reference Table

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Self-Check Questions

1. Both self-defense and necessity are justification defenses. What distinguishes the source of threat each defense responds to, and how would this affect which defense applies in a scenario involving a car accident versus a mugging?

2. A defendant claims they were too drunk to form the intent to commit burglary. Under what circumstances would this defense succeed, and why wouldn't it work for a charge of assault?

3. Compare the insanity defense and duress. Both are excuse defenses, but what type of pressure does each respond to, and which one cannot be used for murder charges?

4. If a defendant is charged with theft but genuinely believed the property belonged to them, which defense applies? How does this differ from a defendant who didn't know taking the item was illegal?

5. An undercover officer repeatedly pressures a person with no criminal history to sell drugs until they finally agree. What defense would apply, and what key factor would determine its success?