The Common Law Elements of a Crime

Before the American Law Institute created the Model Penal Code (MPC), judges, prosecutors, defendants, and defense lawyers alike all relied solely on the common law. The common law is the collection of judge-made rules, tests, definitions, and requirements which have accumulated over the years to form a working body of law. Under this system, each district or jurisdiction could have a different rule, name, or definition of a single term. This eventually led to some confusion. The common law developed a set of elements that must be found in order for an individual to have committed a crime and for that individual to be held liable.

The first of these elements is known as the actus reus. This is the actual action taken by the individual who is suspected of having done something that society dislikes. The actus reus must be voluntary (i.e. the wind cannot have raised a person’s arm which made him strike something) and must exist in order for a charge to move forward.

The next element is the mens rea. This is perhaps the most confusing element and one that the Model Penal Code was established in order to clarify. Here, the defendant or suspect must have the mens rea, or mental state or intent, to have done whatever he or she is suspected of doing in order for the crime to have been committed. Unfortunately, it was often extremely difficult for judges to agree on the exact definition of mens rea, leading to much confusion.

The third element of a common law definition of a crime is causation. The act that was committed by the defendant must have clearly caused whatever happened. The most simple test is the “but for” test. This test asks, “But for the defendant’s actions, would X still have happened?”

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