31 Section 3.4: Legal Defenses

To successfully obtain a conviction, the prosecutor must show all of the elements of the crime beyond a reasonable doubt in criminal court. This is not the end of it in some cases. It must also be shown (if the issue is raised) that the actus reus and the mens rea was present, but also that the defendant committed the act without justification or excuse. Both justifications and excuses are species of legal defenses. If a legal defense is successful, it will either mitigate or eliminate guilt.

justification consists of a permissible reason for committing an act that would otherwise be a crime. Under normal circumstances, for example, it would be a crime to shoot a man dead on the street. If, however, the man was a mugger and had the shooter at knifepoint, then the justification of self-defense could be raised. A justification means that an act would normally be wrong, but under the circumstances it was the right thing to do. An excuse is different.

The Insanity Defense

The term insanity comes from the law; psychology and medicine do not use it. The everyday use of the term can be misleading. If a person acts abnormally, they tend to be considered by many as “crazy” or “insane.” At law, merely having a mental disease or mental defect is not adequate to mitigate guilt. It must be remembered that Jeffery Dahmer was determined to be legally sane, even though everyone who knows the details of his horrible acts knows that he was seriously mentally ill. To use insanity as a legal excuse, the defendant has to show that he or she lacked the capacity to understand that the act was wrong, or the capacity to understand the nature of the act. Some jurisdictions have a not guilty by reason of insanityplea.

The logic of the insanity defense goes back to the idea of mens rea and culpability. We as a society usually only want to punish those people who knew what they were doing was wrong. Most people believe that it is morally wrong to punish someone for an unavoidable accident. Likewise, society does not punish very young children for acts that would be crimes if an adult did them. The logic is that they do not have the maturity and wisdom to foresee and understand the nature of the consequences of the act. Put in oversimplified terms, if a person is so crazy that they do not understand that what they are doing is wrong, it is morally wrong to punish them for it.

Over the years, different courts in different jurisdictions have devised different tests to determine systematically if a criminal defendant is legally insane. One of the oldest and most enduring tests is the M’Naghten rule, handed down by the English court in 1843. The basis of the M’Naghten test is the inability to distinguish right from wrong. The Alabama Supreme Court, in the case of Parsons v. State (1887), first adopted the Irresistible Impulse Test. The basic idea is that some people, under the duress of a mental illness, cannot control their actions despite understanding that the action is wrong.

Today, all of the federal courts and the majority of state courts use the substantial capacitytest developed within the Model Penal Code. According to this test, a person is not culpable for a criminal act “if at the time of the crime as a result of mental disease or defect the defendant lacked the capacity to appreciate the wrongfulness of his or her conduct or to conform the conduct to the requirements of the law.” In other words, this test contains the awareness of wrongdoing standard of M’Naghten as well as the involuntary compulsion standard of the irresistible impulse test.

It is a Hollywood myth that many violent criminals escape justice with the insanity defense. In fact, the insanity defense is seldom attempted by criminal defendants and is very seldom successful when it is used. Of those who do successfully use it, most of them spend more time in mental institutions than they would have spent in prison had they been convicted. The insanity defense is certainly no “get out of jail free card.”


Entrapment is a defense that removes blame from a person who commits a criminal act when convinced to do so by law enforcement. In other words, people have the defense of entrapment available when police lure them into crime. A valid entrapment defense has two related elements: There must be a government inducement of the crime, and the defendant’s lack of predisposition to engage in the criminal conduct. Mere solicitation, however, to commit a crime is not inducement. Inducement requires a showing of at least persuasion or mild coercion.


As a matter of political theory, the right to use force is handed over to the government via the social contract. This power to use force is entrusted to law enforcement. Thus, when force is called for to end a confrontation, people should call the police. There are times, however, when the police are not available in emergencies. In these rare instances, it is permissible for the average citizen to use force to protect themselves and others from violent victimization.

The legality of using force in self-defense hinges on reasonableness. Whether a use of force decision was a reasonable one will always depend on the circumstances of each individual situation. The amount of force used should be the minimum likely to repel the attack. The defense also requires that the danger be imminent. In other words, the use of force cannot be preemptive or retaliatory. Generally, deadly force can only be used to prevent loss of life. Some jurisdictions allow the use of non-deadly force to prevent thefts.


While there is some logic to the idea that being intoxicated diminishes a person’s capacity to develop mens rea, it usually serves to enhance rather than mitigate criminal culpability. There are some jurisdictions that allow voluntary intoxication as a factor that mitigates culpability, such as when murder in the first degree is reduced to murder in the second degree. Involuntary intoxication is another matter. If a defendant has been given a drug without their knowledge, then a defense of involuntary intoxication may be available.


It is often said, “Everybody makes mistakes.” The law recognizes this, and mistake can sometimes be a defense to a criminal charge. Mistakes made because the situation was not really the way the person thought it was are known as mistakes of fact. These can be a criminal defense. Mistakes as to matters of law (mistakes of law) can never be used as a criminal defense. There is a presumption in American law that everyone knows the criminal law. This may seem like a preposterous assumption, but consider the alternative. If a defendant could mount a defense by claiming that he or she did not know the act was criminal, then everyone could commit every crime at least once and get away with it by claiming that they did not know. For this reason, the law has to presume that everybody knows the law.


The defense of necessity is based on the idea that it is sometimes necessary to choose one evil to prevent another, such as when property is destroyed to save lives. The necessity defense is sometimes referred to as the lesser of two evils defense because the evil that he actor seeks to prevent must be a greater harm that the evil that he or she does to prevent it. In most jurisdictions, the defense will not be available if the person created the danger they were avoiding.


Duress , sometimes known as coercion, means that the actor did the criminal act because they were forced to do so by another person by means of a threat. The idea is that while the actor commits the actus reus of the offense, the mens rea element, the criminal intent, was that of the person that coerced the actor to commit the crime. The effect of a successful duress defense is a matter of state law, so may be different in different jurisdictions. Most jurisdictions require that the actor have no part in becoming involved in the situation.

Key Terms

Coercion, Deadly Force, Duress, Entrapment, Excuse, Imminent Danger, Insanity Defense, Involuntary Intoxication, Irresistible Impulse Test, Justification, Lesser of Two Evils Defense, Mistake Defense, Mistake of Face, Mistake of Law, M’Naghten Rule, Necessity Defense, Non-deadly Force, Not Guilty By Reason of Insanity, Parsons v. State(1887), Self-defense, Solicitation, Substantial Capacity Test, Voluntary Intoxication


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