Suppose a ship at sea suddenly develops problems and the passengers are put into life boats. One boat carrying 20 people gets lost. After 8 days without food and water, the passengers decide to kill one of the passengers and eat him. They kill a 7 years old boy and feed on him. When they get back to shore, they are charged with murder. If the passengers plead necessity as a defense, will they succeed?
The stated facts are an almost carbon copy of the facts in R v Dudley and Stephens (1884) 14 QBD 273 DC so we know the answer - No
The court ruled that cannibalising the boy was not urgently necessary. Even though the cabin boy would almost certainly have died of natural causes, the sailors killed the boy intentionally and were guilty of murder. There was some degree of necessity arising from the threat of starvation but, at any moment, a ship could have sailed over the horizon to save them as, indeed, the three were rescued. Since they could never be sure that the killing was actually necessary from one minute to the next, the defence was denied.
Specifically, cannibalism is not the crime; murder is.
an urgent and immediate threat to life which creates a situation in which the defendant reasonably believes that a proportionate response to that threat is to break the law.
In practice, necessity is only a defence to relatively minor crimes, like breaking traffic rules in a race to a hospital or breaking a window to escape a burning building - the consequences of the breach have to be relatively trivial in relationship to the consequences of adhering to the law.
A typical Model Penal Code based provision of the choice of evils defense is as follows:
Part 7 - Justification and Exemptions From Criminal Responsibility
§ 18-1-702. Choice of evils
Universal Citation: CO Rev Stat § 18-1-702 (2016)
(1) Unless inconsistent with other provisions of sections 18-1-703 to 18-1-707, defining justifiable use of physical force, or with some other provision of law, conduct which would otherwise constitute an offense is justifiable and not criminal when it is necessary as an emergency measure to avoid an imminent public or private injury which is about to occur by reason of a situation occasioned or developed through no conduct of the actor, and which is of sufficient gravity that, according to ordinary standards of intelligence and morality, the desirability and urgency of avoiding the injury clearly outweigh the desirability of avoiding the injury sought to be prevented by the statute defining the offense in issue.
(2) The necessity and justifiability of conduct under subsection (1) of this section shall not rest upon considerations pertaining only to the morality and advisability of the statute, either in its general application or with respect to its application to a particular class of cases arising thereunder. When evidence relating to the defense of justification under this section is offered by the defendant, before it is submitted for the consideration of the jury, the court shall first rule as a matter of law whether the claimed facts and circumstances would, if established, constitute a justification.
It is hypothetically recognized under U.S. law, but rarely applied in circumstances like the ones described.