Criminal Law

Criminal Law, as distinguished from civil law, is a system of laws concerned with punishment of individuals who commit crimes. Thus, where in a civil case two individuals dispute their rights, a criminal prosecution involves the people as a whole deciding whether to punish an individual for his conduct or lack of conduct (i.e. omission). Just as the people decide what conduct to punish, so the people decide what punishment is appropriate. Accordingly, punishments vary with the severity of the offense—from a simple fine (e.g. for a traffic violation) to loss of freedom (e.g. for murder).

Each state decides what conduct to designate a crime. Thus, each state has its own criminal code. Congress has also chosen to punish certain conduct, codifying federal criminal law in Title 18 of the U.S. Code. Criminal laws vary significantly among the states and the Federal Government. While some statutes resemble the common law criminal code, others, like the New York Penal Law, closely mimic the Model Penal Code (MPC).

A “crime” is any act or omission in violation of a law prohibiting it, or omitted in violation of a law ordering it. The government cannot prosecute an individual for conduct that was not declared criminal at the time the individual acted. The Constitution explicitly forbids in Article 1, Sections 9 and 10 retroactively applicable criminal laws—ex post facto laws.

It is also important to note that a law cannot punish a person simply for their status. As the Supreme Court explained in Robinson v. California, 370 U.S. 660 (1962), any statute that criminalizes the status of a person inflicts a cruel and unusual punishment in violation of the Eight and Fourteenth Amendments. For example, a state could not punish an individual for “being homeless,” which would be a status offense, but could punish a homeless individual for trespassing or loitering, which involves some conduct.


In general, Criminal Law asks and answers three questions:

  1. Did an individual commit a crime?
  2. Which crime did an individual commit?
  3. Does the individual have a defense?


An individual commits a crime if he or she acts in a way that fulfills every element of an offense. The statute establishing the offense also establishes the elements of the offense. In general, every crime involves three elements: first, the act or conduct (“actus reus”); second, the individual’s mental state at the time of the act (“mens rea”); and third, the causal link between the act and the offense. In a criminal prosecution, the government has the burden of proof to establish every element of a crime beyond a reasonable doubt; and third, the individual’s conduct must be the cause of the crime.

The actus reus is the voluntary act that either is in itself wrongful or leads to a wrongful result. For example, driving while drunk is an act that is in itself wrongful. Conduct that causes the death of another, on the other hand, is only wrongful if it leads to a specific result—the death of another. Thus, some laws only punish conduct (i.e. conduct crimes), while others punish the result (i.e. result crimes). In every case, an individual is only guilty of a crime if he or she voluntarily acted. This means that an individual is not guilty if he or she acted on reflex, under hypnosis, or due to a convulsion. In certain circumstances, however, an individual could be found guilty for not acting at all (i.e. failing to act). In other words omission can serve to establish the actus reus if an individual was under a duty to act and failed to discharge that duty.

Regardless of whether an individual is being prosecuted for an affirmative act or an omission, an individual is only guilty if he or she had the requisite mental state (“mens rea”) when engaging in an act or an omission. The statute codifying the crime typically prescribes the mental state requisite for a prosecution. For example:

  • If a statute defined burglary as breaking into the dwelling house of another with intent to commit felony therein, an individual could only be found guilty of burglary if the prosecution could establish that the individual intended to commit a felony.
  • If a statute defined murder as knowingly causing the death of another, an individual could only be found guilty of murder if he or she knew that his or her conduct would cause the death of another.
  • If a statute defined involuntary manslaughter as negligently causing the death of another, an individual could only be found guilty of involuntary manslaughter if he or she caused the death of another by failing to exercise that level of care that a reasonable person would.

When a statute does not prescribe a specific mental state, a court will read in “that mens rea which is necessary to separate wrongful from innocent conduct.”

Even if an individual acted with an unlawful mental state, an individual cannot be convicted if he caused no crime. Of course, if the act itself is criminal, then the “cause” requirement is satisfied the moment the individual acts. When a statute punishes the result of some conduct, however, the prosecution must prove that the conduct was the actual cause and the legal cause of the unlawful result. In other words, the prosecution must prove that the conduct was the but-for and proximate cause of the result