i read Cannibalism: A Perfectly Natural History. these are my own notes, so take with a grain of salt, some fava beans and a nice chianti.

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A N I M A L S

reasons:

• not recognising their conspecifics (= animals of the same species)
• stressful environments, incl. lack of other food
• prioritizing fewer but strong over more but weaker offspring

types:

• filial cannibalism (eating one's own offspring)
  • producing eggs just for food (insects, fish)
  • eating eggs for food because there's so many anyway (fish)
  • eating eggs as food while protecting the eggs (fish)
• sibling cannibalism
  • in-utero ('intrauterine') (sand tiger sharks)
  • oophagy (eating siblings who are still in eggs)
  • sacrificing one offspring to feed the others (birds?)
• sacrificing oneself as food for one's young (black lace-weaver spider)
• sexual cannibalism (of the smaller partner, which in invertebrates is mostly the male) (praying mantis)
• heterocannibalism (eating unrelated conspecifics)

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H U M A N S

• two types:
  • exocannibalism (outside of own social group)
  • endocannibalism (usually ritual consumption of the deceased)
• 'carribean' and 'cannibal' share a word origin! The 'caribs' (indigenous people of the caribbean), later called 'canibs' (probably because of "canine-like faces") were supposedly cannibals.

survival cannibalism:

• the donner party (1847)
• the siege of leningrad (1941-42)
• the great chinese famine during the 'great leap forward' (1959-61)

ritualistic/cultural cannibalism:

• transsubstantiation of christianity
  • (= the conversion of the substance of the Eucharistic elements into the body and blood of Christ at consecration, only the appearances of bread and wine still remaining) ;
  • the church - to this day - says it's the actual body and blood of Christ, not just a metaphor.
  • many jews were killed for supposedly 'abusing' the body of Christ by stabbing bread etc.
• mourning : less mourning when body is gone + Disrespectful/degrading to the dead to put them in the cold, wet dirt
• medicinal cannibalism in europe and china
  • human blood, skull powder, organs, fingernails, ...
  • mum(m)ia (mummy powder)
• filial piety in china: children offer their parents/elders parts of their body out of respect
• placentophagy (eating of the placenta): not common at all throughout humanity's history, but currently a trend of moms in the west. supposedly gives you nutrients and energy; no scientific evidence yet.
• the fore people of papua new guinea

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what is and isn't cannibalism depends on your definition. is scavenging cannibalism? is eating your placenta? breastfeeding? chewing your nails?