In the United States, the principle of dual sovereignty applies to homicide, as to other crimes. If murder is committed within the borders of a state, that state has jurisdiction. If the victim is a federal official, an ambassador, consul or other foreign official under the protection of the United States, or if the crime took place on federal property or involved crossing state borders, or in a manner that substantially affects interstate commerce or national security, then the Federal Government also has jurisdiction. If a crime is not committed within any state, then Federal jurisdiction is exclusive: examples include the District of Columbia, naval or US-flagged merchant vessels in international waters, or a military base abroad. In cases where a murder involves both state and federal jurisdiction, the offender can be tried and punished separately for each crime without raising issues of double jeopardy.

Modern codifications tend to create a genus of offenses, known collectively as homicide, of which murder is the most serious species, followed by manslaughter which is less serious, and ending finally in justifiable homicide, which is not a crime at all. Because there are 51 jurisdictions, each with its own criminal code, this section treats only the crime of murder, and does not deal with state-by-state specifics.

Murder

At base, murder consists of an intentional unlawful act with a design to kill and fatal consequences. Generally, an intention to cause great bodily harm is considered indistinguishable from an intention to kill, as is an act so inherently dangerous that any reasonable person would realize the likelihood of fatality. Thus, if the defendant hurled the victim from a bridge, it is no defense to argue that harm was not contemplated, or that the defendant hoped only to break bones.

The intent to commit murder is often called malice aforethought, and can be inferred when the defendant commits an act that shows depraved indifference to human life, or (in federal court and those states that apply the felony murder rule) whenever a victim is killed during the commission of another felony, whether or not the defendant intended the killing, or even committed the fatal act. In this case, the intention to place the victim in great bodily harm is inferred from the defendant's intent to commit the felony. Some states also require the underlying felony to be an 'inherently' dangerous one.

Degrees of murder

Before the famous case of Furman v. Georgia, most states distinguished two degrees of murder. While the rules differed by state, a reasonably common scheme was that of Pennsylvania, passed in 1794: "Murder which shall be perpetrated by means of poison, or by lying in wait, or by any other kind of wilful, deliberate, and premeditated killing, or which shall be committed in the perpetration or attempt to perpetrate, any arson, rape, robbery, or burglary, shall be deemed murder of the first degree; and all other kinds of murder shall be deemed murder of the second degree."[7] "Murder one", as the term was popularized by novels and television, carried a penalty of death, or life in prison, while the penalty for "murder two" was generally around 20 years in prison.

After the Supreme Court placed new requirements on the imposition of the death penalty, most states adopted one of two schemes. In both, third degree murder became the catch-all, while first degree murder was split. The difference was whether some or all first degree murders should be eligible for the most serious penalty (generally death, but sometimes life in prison without the possibility of parole.).

* The first scheme, used by New York among other states:

1. First Degree Murder: Murder involving special circumstances, such as murder of a police officer, judge, fireman or witness to a crime, multiple murder, the use of torture or especially heinous means, or means requiring great preparation, such as poison or lying in wait.
2. Second Degree Murder: Any premeditated murder or felony murder that does not involve special circumstances.
3. Third Degree Murder: All other murder.

* The second scheme, used by Pennsylvania among other states:

1. First Degree Murder: All premeditated murders, and (in some states) murders involving certain especially dangerous felonies, such as arson or rape, or committed by an inmate serving a life sentence.
2. Second Degree Murder: Any felony murder not a first degree murder.
3. Third Degree Murder: All other murder.

Some states, such as California, simply preserved the old distinction and have no offense called third degree murder. Other states use the term "capital murder" for those offenses that merit death, and the term is often used even in states whose statutes do not include the term. As of 2006, 38 states and the federal government have laws allowing capital punishment for certain murders and related crimes (such as treason and terrorism). The penalty is rarely asked for and more rarely imposed, but it has generated tremendous public debate.

Voluntary and involuntary manslaughter

Within the law of homicide, manslaughter, also called criminally negligent homicide in the United States, is a less serious offense than murder where one person has killed another.

Manslaughter is usually broken into two categories:

* Voluntary manslaughter

This arises in cases where the defendant may have an intent to cause death or serious injury, but the potential liability for murder is mitigated by the application of a defense. For example, the defendant may be provoked into a loss of control by unexpectedly finding a spouse in the arms of a lover or witnessing an attack against his or her child, or the defendant may have diminished capacity.

* Involuntary manslaughter

This is where death occurs due to recklessness or criminal negligence with no intention to kill or cause serious injury. In Northern Ireland, Scotland, and the United States, the law differentiates between levels of fault based on the mens rea (Latin for a "guilty mind").

Misdemeanor manslaughter

In the United States, this is a lesser version of felony murder and covers a person who causes the death of another while committing a misdemeanor: a violation of the law that does not rise to the level of a felony. This may automatically lead to a conviction for the death, if the misdemeanor involved a law designed to protect human life. Many safety laws are infractions, meaning that a person can be convicted regardless of mens rea. Vehicular manslaughter is a kind of misdemeanor manslaughter, which holds persons liable for any death that occurs because of a violation of traffic safety laws.