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The mens rea requirement depends on the nature of the crime and all the circumstances surrounding the act. In general, though, the requirement means that the accused must in some way have intended the criminal consequences of his act. Suppose, for example, that Charlie gives Gabrielle a poison capsule to swallow. That is the act. If Gabrielle dies, is Charlie guilty of murder? The answer depends on what his state of mind was. Obviously, if he gave it to her intending to kill her, the act was murder.
What if he gave it to her knowing that the capsule was poison but believing that it would only make her mildly ill? The act is still murder, because we are all liable for the consequences of any intentional act that may cause harm to others. But suppose that Gabrielle had asked Harry for aspirin, and he handed her two pills that he reasonably believed to be aspirin (they came from the aspirin bottle and looked like aspirin) but that turned out to be poison, the act would not be murder, because he had neither intent nor a state of knowledge from which intent could be inferred.
Not every criminal law requires criminal intent as an ingredient of the crime. Many regulatory codes dealing with the public health and safety impose strict requirements. Failure to adhere to such requirements is a violation, whether or not the violator had mens rea. The United States v. Park case, Section 6.7 "Cases", a decision of the US Supreme Court, shows the different considerations involved in mens rea.
Ordinarily, ignorance of the law is not an excuse. If you believe that it is permissible to turn right on a red light but the city ordinance prohibits it, your belief, even if reasonable, does not excuse your violation of the law. Under certain circumstances, however, ignorance of law will be excused. If a statute imposes criminal penalties for an action taken without a license, and if the government official responsible for issuing the license formally tells you that you do not need one (though in fact you do), a conviction for violating the statute cannot stand. In rare cases, a lawyer’s advice, contrary to the statute, will be held to excuse the client, but usually the client is responsible for his attorney’s mistakes. Otherwise, as it is said, the lawyer would be superior to the law.
Ignorance or mistake of fact more frequently will serve as an excuse. If you take a coat from a restaurant, believing it to be yours, you cannot be convicted of larceny if it is not. Your honest mistake of fact negates the requisite intent. In general, the rule is that a mistaken belief of fact will excuse criminal responsibility if (1) the belief is honestly held, (2) it is reasonable to hold it, and (3) the act would not have been criminal if the facts were as the accused supposed them to have been.
One common technique of criminal investigation is the use of an undercover agent or decoy—the policeman who poses as a buyer of drugs from a street dealer or the elaborate “sting” operations in which ostensibly stolen goods are “sold” to underworld “fences.” Sometimes these methods are the only way by which certain kinds of crime can be rooted out and convictions secured.
But a rule against entrapmentWhen a police officer or other government agent entices people to commit crimes they were not disposed to commit without the government agent’s suggestions and inducements. limits the legal ability of the police to play the role of criminals. The police are permitted to use such techniques to detect criminal activity; they are not permitted to do so to instigate crime. The distinction is usually made between a person who intends to commit a crime and one who does not. If the police provide the former with an opportunity to commit a criminal act—the sale of drugs to an undercover agent, for example—there is no defense of entrapment. But if the police knock on the door of one not known to be a drug user and persist in a demand that he purchase drugs from them, finally overcoming his will to resist, a conviction for purchase and possession of drugs can be overturned on the ground of entrapment.
A number of other circumstances can limit or excuse criminal liability. These include compulsion (a gun pointed at one’s head by a masked man who apparently is unafraid to use the weapon and who demands that you help him rob a store), honest consent of the “victim” (the quarterback who is tackled), adherence to the requirements of legitimate public authority lawfully exercised (a policeman directs a towing company to remove a car parked in a tow-away zone), the proper exercise of domestic authority (a parent may spank a child, within limits), and defense of self, others, property, and habitation. Each of these excuses is a complex subject in itself.
A further defense to criminal prosecution is the lack of mental capacity to commit the crime. Infants and children are considered incapable of committing a crime; under common law any child under the age of seven could not be prosecuted for any act. That age of incapacity varies from state to state and is now usually defined by statutes. Likewise, insanity or mental disease or defect can be a complete defense. Intoxication can be a defense to certain crimes, but the mere fact of drunkenness is not ordinarily sufficient.
In the United States, some crimes can be committed by not following strict regulatory requirements for health, safety, or the environment. The law does provide excuses from criminal liability for mistakes of fact, entrapment, and lack of capacity.