Human cannibalism
Institutionalized, survival, and pathological cannibalism

One major distinction is whether cannibal acts are accepted by the culture in which they occur – institutionalized cannibalism – or whether they are merely practised under starvation conditions to ensure one's immediate survival – survival cannibalism – or by isolated individuals considered criminal and often pathological by society at large – cannibalism as psychopathology or "aberrant behavior". Institutionalized cannibalism, sometimes also called "learned cannibalism", is the consumption of human body parts as "an institutionalized practice" generally accepted in the culture where it occurs.

By contrast, survival cannibalism means "the consumption of others under conditions of starvation such as shipwreck, military siege, and famine, in which persons normally averse to the idea are driven by the will to live". Also known as famine cannibalism, such forms of cannibalism resorted to only in situations of extreme necessity have occurred in many cultures where cannibalism is otherwise clearly rejected. The survivors of the shipwrecks of the Essex and Méduse in the 19th century are said to have engaged in cannibalism, as did the members of Franklin's lost expedition and the Donner Party.

Such cases often involve only necro-cannibalism (eating the corpse of someone already dead) as opposed to homicidal cannibalism (killing someone for food). In modern English law, the latter is always considered a crime, even in the most trying circumstances. The case of R v Dudley and Stephens, in which two men were found guilty of murder for killing and eating a cabin boy while adrift at sea in a lifeboat, set the precedent that necessity is no defence to a charge of murder. This decision outlawed and effectively ended the practice of shipwrecked sailors drawing lots in order to determine who would be killed and eaten to prevent the others from starving, a time-honoured practice formerly known as a " custom of the sea ".

In other cases, cannibalism is an expression of a psychopathology or mental disorder, condemned by the society in which it occurs and "considered to be an indicator of severe personality disorder or psychosis". Well-known cases include Albert Fish, Issei Sagawa, and Armin Meiwes. Fantasies of cannibalism, whether acted out or not, are not specifically mentioned in manuals of mental disorders such as the DSM, presumably because at least serious cases (that lead to murder) are very rare.
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