Principles of Criminal Liability (Criminal Law Summary)

Elements of a Crime: A crime generally has two components: the actus reus, the physical or external part, and the mens rea, the mental or internal feature. The actus reus generally includes a voluntary act that causes social harm. Causation links the voluntary act to the social harm. The requirement of a voluntary act is generally an implicit element of criminal statutes supported by common law. In exceptional cases, an omission (failure to act when there is a legal duty) can serve as the basis for criminal responsibility. A duty to act can arise from common law, statute, or contract. Most human acts are considered voluntary, with involuntary acts including reflexive actions, spasms, seizures, and movements during unconsciousness or sleep.

Mens rea refers to criminal intent or a "guilty mind". It is the state of mind statutorily required to convict a defendant of a particular crime. The prosecution typically must prove mens rea beyond a reasonable doubt. The Model Penal Code (MPC) categorizes culpable mental states into four levels: purposely, knowingly, recklessly, and negligently.

Concurrence (or contemporaneity) is the need to prove that the actus reus and mens rea occurred simultaneously, except in strict liability crimes. The single transaction principle allows a sequence of inevitably linked events to be viewed as one transaction, where mens rea formed at any point during the sequence can suffice.

Classification of Crimes: Crimes can be classified in several ways, most commonly as felonies (punishable by death or imprisonment for more than one year) and misdemeanors (lesser offenses usually punishable by a fine or incarceration for less than one year). Crimes are also categorized as inchoate offenses and strict liability offenses. At common law, there were nine major felonies and various misdemeanors.

Inchoate Offenses: Inchoate offenses (attempt, solicitation, and conspiracy) involve steps taken toward committing another crime, even if the final harmful result does not occur. Attempt involves preparatory conduct that comes dangerously close to success (common law proximity test) or constitutes a substantial step strongly corroborative of criminal purpose (MPC substantial step test). Solicitation occurs when one intentionally entices another to commit a crime. Conspiracy is an agreement between two or more persons to commit an unlawful act, coupled with an overt act in furtherance of the agreement. The Pinkerton doctrine can hold a conspirator liable for foreseeable crimes committed by co-conspirators. The merger doctrine often prevents conviction for both an attempt or solicitation and the completed offense, but conspiracy typically does not merge. Renunciation or withdrawal may be a defense to inchoate offenses in some jurisdictions.

Specific Crimes: Homicide is the unlawful killing of a human being, categorized into murder, manslaughter, and sometimes criminally negligent homicide. Murder is typically defined as unlawful killing with malice aforethought. US law for murder varies by jurisdiction, often with degrees of murder (first, second, sometimes third) and different classifications in the Model Penal Code (purposely/knowingly, reckless, negligent). Property crimes include larceny, embezzlement, false pretenses, robbery, burglary, and arson, each involving different ways property rights are violated.

Defenses to Criminal Liability: Defenses are broadly divided into justifications (admitting actus reus and mens rea but claiming the act was legally permissible) and excuses (conceding the act was wrongful but arguing the actor should not be held criminally responsible). Justification defenses include self-defense, defense of others, defense of property, necessity, and law enforcement privilege. Excuse defenses include insanity, intoxication, duress, entrapment, and mistake.

The insanity defense concerns the defendant's state of mind, potentially negating mens rea, with various legal tests like the M'Naghten Rule, Durh