In 1977 Michael Harner published an argument in American Ethnologist that Mexica sacrifice, and specifically the ritual consumption of the bodies of the sacrificed, was best explained by ecology. The Valley of Mexico, on this account, had been stripped of large domesticable herbivores; the population had grown past what its protein supply could carry; and an institution that killed people and distributed their flesh was a response to that pressure, dressed in religion. Marvin Harris took the argument to a general readership the same year in Cannibals and Kings, and it travelled far beyond anthropology.
The argument had real appeal. It was materialist where the alternative explanations were theological, it was quantitative in form, and it appeared to explain a practice that had otherwise been described mainly with adjectives. It also had a shape that recurs in writing about non-European religion: the participants are taken to be mistaken about their own reasons, and the analyst supplies the true ones.
It is recorded here as a study rather than as a fact because it is a clean example of a testable proposition about sacrifice. It made claims about calories, about who ate what, and about when in the agricultural year the practice concentrated. Claims of that kind can be checked, and within a year they were.