For an act to be a crime, it must be not only intentional and in violation of a criminal law, but also without defense or justification. Defense
refers to situations that can mitigate guilt in a criminal case. Two common defenses are insanity and entrapment. Justification
is any just cause for committing an act that otherwise would be a crime. Self‐defense is a prime example.
Insanity is a legal term, not a medical term. It refers to any unsoundness of mind, mental defect, or lack of reason that prevents people from distinguishing right from wrong and from understanding the consequences of their actions. Guilty defendants can be found “not guilty by reason of insanity” because of the belief that people should be punished for their crimes only if they could control their behavior and knew what they were doing was wrong. Otherwise, it is improper to hold people morally accountable for criminal behavior.
Courts use several tests of insanity. Under the M'Naghten rule (1843), defendants are considered not guilty by reason of insanity if at the time of the crime they were unable to distinguish right from wrong. All federal courts and about half of the state courts have the Model Penal Code's substantial capacity test. A person is not responsible for criminal conduct “if at the time of such conduct as a result of a mental disease or defect he or she lacks substantial capacity either to appreciate the criminality of his or her conduct or to conform his or her conduct to the requirements of law”. This test provides a broader, more encompassing definition of insanity than the M'Naghten rule. Defendants pleading insanity under the substantial capacity test have to show only that they are mostly unable to function mentally.
The Comprehensive Crime Control Act (1984) changed the federal rules on the insanity defense, limiting it to those who are unable to understand the wrongfulness of their acts as a result of severe mental disease. This act shifted the burden of proof to the defense. Now, the defense must prove beyond a reasonable doubt that the defendant was insane at the time of the crime. Under this law, a person who is found not guilty only by reason of insanity must be committed to a mental hospital until he or she no longer poses a threat to society. Many state courts have adopted these rules.
A common misperception about the insanity defense is that it allows many violent criminals to escape punishment for their acts. Studies show that the insanity plea is used in fewer than one percent of serious criminal cases and is rarely successful. When it succeeds, offenders generally spend more time in mental institutions than they would have spent in prison if they had been convicted.
Entrapment is a legal defense that sets free the suspect who has been induced by law enforcement agents to commit a crime. The U.S. Supreme Court invented the entrapment doctrine to control outrageous, overreaching police activity that endangers civil liberties and violates fundamental fairness. The Court's subjective test, which most state courts and all federal courts follow, holds that entrapment occurs when a law enforcement agent puts a criminal idea into the mind of an innocent person who otherwise would not have committed the offense. The focus is on the predisposition of the defendant: Did the idea to commit the crime originate with the defendant or law enforcement?
A person defending himself or herself can use only reasonable force in self‐defense. How much force is reasonable depends on the circumstances of each situation. The force used to repel an attack should be proportionate to the amount of force used against the defendant. To use this defense, the danger must be imminent and the defendant must look for alternative ways of avoiding the danger. The rules of self‐defense also apply to defense of another and the defense of property. Defendants can support a self‐defense argument with evidence that a victim had a history of violence, and the prosecution can produce evidence that the victim was not prone to violence. Some states also allow the prosecution to offer evidence of the defendant's history of violence.