Criminal Law: Defenses

Criminal liability is not only about proving that a person committed a prohibited act with the required mental state. A complete analysis also asks whether a recognized defense defeats or reduces liability. In criminal law, defenses generally fall into two broad categories: justifications and excuses. A justification argues that the conduct was the right or permissible response under the circumstances. An excuse concedes the conduct was wrong but claims the actor should not be held blameworthy because of a condition or pressure that undermines responsibility.

Understanding the difference matters in practice. A justified act, such as lawful self-defense, is treated as socially acceptable. An excused act, such as conduct committed during legal insanity, is still harmful and unlawful, but the actor is not held criminally culpable in the usual way.

Justification Defenses

Self-Defense

Self-defense is the most familiar justification. It permits the use of force to protect oneself from an unlawful threat. The core requirements are straightforward in principle and fact-intensive in application:

• Imminence: The threat must be immediate, not a future possibility or a past grievance.
• Necessity: Force must be needed to avert the threatened harm; if a safe alternative exists, the defense may fail depending on the jurisdiction and circumstances.
• Proportionality: The level of force must match the threat. Deadly force is typically justified only to prevent death, serious bodily injury, or comparable grave harm.
• Reasonableness: Many systems ask whether the defendant’s beliefs and actions were reasonable under the circumstances, not merely sincere.

A classic example is a person who is about to be attacked with a knife and uses force to stop the assault. By contrast, a person who uses deadly force in response to a shove or insult will usually fail the proportionality requirement.

Self-defense often turns on what the defendant reasonably perceived at the moment. Evidence such as the aggressor’s behavior, prior threats, the physical setting, and any disparity in size or strength can be pivotal.

Defense of Others

Defense of others extends the logic of self-defense: a person may use force to protect someone else from unlawful harm. The same basic limits apply, especially imminence, necessity, and proportionality. The difficult questions tend to involve mistakes: what if the intervener reasonably but mistakenly believes the third person is under attack, or misjudges who the aggressor is?

Many jurisdictions permit the defense if the defendant’s belief was reasonable, even if mistaken, because the justification rests on protecting against what appeared to be an imminent unlawful threat. Practically, this doctrine encourages intervention in emergencies while still limiting vigilante behavior by demanding reasonable judgment and proportional force.

Necessity

Necessity is a justification sometimes described as choosing the lesser evil. It applies when breaking the law is necessary to prevent a greater harm. The usual themes include:

• The harm avoided must be significant and imminent.
• The defendant must reasonably believe the illegal act will avert the harm.
• No adequate legal alternative must exist.
• The response must be proportionate: the harm caused by the illegal act must be less than the harm prevented.

Necessity is highly context-dependent. It is easier to grasp through common-sense scenarios: entering private property to escape a life-threatening storm, or breaking a window to rescue someone from a burning building. The defense is not a general license to disregard the law based on personal policy views. Courts typically scrutinize whether the situation truly left no lawful option and whether the defendant’s response was constrained to what the emergency required.

Excuse Defenses

Insanity

Insanity is an excuse defense. It does not claim the act was justified; it claims that a mental condition so impaired the defendant’s capacity that criminal blame is not appropriate. Two prominent legal tests are the M'Naghten rule and the Model Penal Code (MPC) standard.

The M'Naghten Rule

Under the M'Naghten approach, the focus is cognitive. The question is whether, because of a mental disease or defect, the defendant either:

• did not know the nature and quality of the act, or
• did not know the act was wrong.

This test concentrates on knowledge and understanding at the time of the offense. It can be demanding because many defendants with serious mental disorders still have some awareness of what they are doing and that society condemns it.

The Model Penal Code (MPC) Test

The MPC test is broader and combines cognitive and volitional elements. It asks whether, due to mental disease or defect, the defendant lacked substantial capacity either:

• to appreciate the criminality (wrongfulness) of the conduct, or
• to conform conduct to the requirements of law.

Two phrases matter: “substantial capacity” and “appreciate.” The MPC does not require total incapacity, and “appreciate” suggests a deeper, meaningful understanding rather than mere factual awareness. It also recognizes that some disorders may impair self-control in a legally significant way.

Insanity is not the same as mental illness. Many people with diagnosed conditions are still legally responsible. The defense typically requires expert testimony, detailed behavioral evidence, and careful attention to the defendant’s functioning at the time of the act.

Duress

Duress excuses criminal conduct when the defendant commits an offense because of an immediate threat of serious harm. The law’s premise is that the coercion overwhelms ordinary free choice. Key elements typically include:

• A threat of death or serious bodily injury to the defendant (and in many cases, to another person).
• Imminence and credibility of the threat.
• No reasonable opportunity to escape or seek help.
• A causal connection between the threat and the criminal act.

Duress commonly arises when one person forces another to participate in a crime under threat of violence. The doctrine is limited: it generally does not excuse a calculated, avoidable decision, and jurisdictions often restrict its use for certain grave crimes. The underlying moral judgment is that the law can excuse yielding to extreme coercion, but only when the pressure is immediate and truly inescapable.

Procedural and Conduct-Based Defenses

Entrapment

Entrapment addresses improper government inducement. It is designed to prevent law enforcement from manufacturing crime by persuading an otherwise law-abiding person to offend. While details vary by jurisdiction, the concept centers on two ideas:

• Inducement: Government agents or informants must have actively encouraged or pressured the defendant to commit the offense, beyond merely providing an opportunity.
• Predisposition: Many courts ask whether the defendant was predisposed to commit the crime before government involvement.

In practical terms, offering someone a chance to buy illegal drugs is usually considered providing an opportunity, not entrapment. Repeated badgering, exploitation of vulnerabilities, or extraordinary promises can move the case toward improper inducement. Entrapment is not a defense for someone who was ready and willing to commit the crime and simply took advantage of a chance presented by police.

How Defenses Work in a Criminal Case

Criminal defenses do not operate in a vacuum. They are evaluated against the prosecution’s proof of the offense and the evidence presented at trial. In many systems, once a defendant produces enough evidence to raise a defense, the prosecution must disprove it beyond a reasonable doubt. In others, certain defenses require the defendant to prove specific elements. The allocation matters because defenses often turn on disputed facts: what the defendant perceived, what alternatives existed, how immediate the threat was, or how severe a mental impairment proved to be.

Defenses also shape charging decisions, plea negotiations, and trial strategy. A self-defense claim can lead to focusing on forensic reconstruction and witness credibility. An insanity defense typically requires extensive mental health evidence and careful legal framing under M'Naghten or the MPC. Duress and necessity often turn on timelines and available alternatives. Entrapment frequently becomes a battle over recorded communications and the defendant’s prior conduct.

Conclusion

Criminal law defenses reflect the legal system’s effort to align punishment with moral blameworthiness and social necessity. Justifications like self-defense, defense of others, and necessity recognize that some unlawful acts are the right response to urgent threats or emergencies. Excuses like insanity and duress acknowledge that, in limited circumstances, a person’s capacity or freedom to choose can be so compromised that criminal punishment is not warranted. Entrapment ensures that the government’s role in policing crime does not cross into creating it.

Taken together, these doctrines show that criminal liability is not just about what happened, but why it happened, what options existed, and whether the law should treat the conduct as permissible, understandable, or punishable.