criminal law

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Criminal law, as distinguished from civil law, is a system of laws concerned with crimes and the punishment of individuals who commit crimes. Thus, where in a civil case two parties dispute their rights, a criminal prosecution involves the government deciding whether to punish an individual for either an act or an omission.

A “crime” is any act or omission in violation of a law prohibiting said action or omission.

Criminal Codes

Each state decides what conduct to designate a crime. As such, each state has its own criminal codeCongress 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).

Codification of Criminal Procedure

Congress codified the federal criminal law and criminal procedure in Title 18 of the U.S. Code with §§ 1 to 2725 dealing with crimes. Title 18 designates various conduct as federal crimes, such as arson, use of chemical weaponscounterfeit and forgeryembezzlementespionagegenocide, and kidnapping. These statutes usually prescribe a maximum sentence appropriate for a convicted individual. For additional Federal Regulations, consult 28 C.F.R.

The federal government has also codified the specific procedures which must take place during a criminal proceeding in the Federal Rules of Criminal Procedure.

Elements of a Crime

An individual commits a crime if they act 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 causation between the act and the effect (typically either proximate causation or but-for causation). In a criminal prosecution, the government has the burden of proof to establish every element of a crime beyond a reasonable doubt.

According to the Supreme Court in Elonis v. United States, 575 U.S. (2015), when a statute does not prescribe a specific mental state, a court will infer the “mens rea which is necessary to separate wrongful from innocent conduct.”

Types of Crimes

Crimes can be generally separated into four categories: feloniesmisdemeanorsinchoate offenses, and strict liability offenses.

Each state, and the federal government, decides what sort of conduct to criminalize. At common law, there were nine major felonies (MurderRobberyManslaughterRapeSodomyLarcenyArsonMayhem, and Burglary) and various misdemeanors (i.e. assaultbatteryfalse imprisonmentperjury, and intimidation of jurors).

The U.S. Code is far more extensive than the common law. Nonetheless, Congress has limited power to make criminal laws. As this power is generally reserved to the states, state criminal codes, such as the New York Penal Law, are far more complicated than the U.S. Code. The N.Y. Penal Law prescribes nine levels of felonies, ranging from residential mortgage fraud in the fourth degree to terrorism.

Sentencing Guidelines

The federal government and state governments have created various sentencing guidelines. Federal courts use the Federal Sentencing Guidelines, while state courts will look at state-specific sentencing guidelines.

Liability for Accomplices 

When multiple parties are involved, the traditional first step is to classify the participants according to the following categories:

  • Principal in the first degree – those who actually commit a crime (i.e., the perpetrator).
    • Perpetrators are not accomplices, and this section does not pertain to them.
  • Principal in the second degree – those who aided, counseled, commanded, or encouraged the perpetrator in the actual commission of a crime.
    • An abettor is considered an accomplice.
  • Accessory before the fact – those who aided, counseled, commanded, or encouraged the perpetrator to commit the crime, without actually being present at the moment of perpetration.
    • An accessory (before the fact) is considered an accomplice.
  • Accessory after the fact – those who aid an individual, knowing the individual to be a criminal, in an effort to hinder the individual’s detection, arrest, trial, or punishment.
    • Accessories (after the fact) are guilty of a separate crime, so this section does not pertain to them.

To convict an accomplice, the prosecutor needs to establish the requisite actus reus and mens rea. That is, the prosecutor must prove that the accomplice acted in support of the perpetrator and had the requisite mental state while doing so. It is important to note that some jurisdictions allow accomplices to be prosecuted independently of the principal perpetrator. Thus, an accomplice could be found guilty of a more severe offense than the principal. In certain jurisdictions, an accomplice may be convicted while the alleged perpetrator is acquitted.

Ex Post Facto

An ex post facto law retroactively punishes actions. The Constitution explicitly forbids this practices in Article 1, Sections 9 and 10.

Punishing For Status

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 Eighth Amendment and Fourteenth Amendment.

  • 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.


There are several defenses available to a defendant in a criminal prosecution. The following list illustrates some common defenses individuals rely on:

  • Failure of Proof – an individual’s simplest defense in a criminal prosecution is to claim that the prosecution has not or cannot prove an element of the offense.
  • Mistakes – in certain circumstances, an individual’s mistake can be used as a defense.
    • Mistake of Law – a mistake regarding the legal status or effect of some situation.
    • Mistake of Fact – a mistake regarding the facts of some situation.
  • Justifications – these are complete defenses
    • Self-Defense: the use of force to protect oneself from an attempted injury by another.
    • Defense of property: a person may use force to protect their property from a felony occurring within.
    • Defense of Others: the right of a person to protect a third party with reasonable force against an assailant who seeks to inflict force upon the third party.
    • Necessity: sometimes referred to as the “choice of evils,” the necessity defense allows an individual to engage in otherwise unlawful conduct if by doing so the individual avoids a greater harm.
  • Excuses – these are partial defenses
    • Duress: an individual may plead duress if another individual forced them to engage in the illegal conduct by force or threat of force.
    • Intoxication: an individual who was involuntarily intoxicated can plead intoxication as a defense to every crime.
      • An individual who was voluntarily intoxicated can plead intoxication as a defense only to crimes that require a specific mental state.
    • Insanity: an insane individual cannot form the requisite mental state, and thus cannot be found guilty.

Further Reading:

For more on criminal law, see this Florida State University Law Review article, this Harvard Law Review article, and this Boston College International and Comparative Law Review article.

[Last updated in August of 2022 by the Wex Definitions Team]