Researchers have uncovered a dark role played by religious rituals in the evolution of complex modern societies. According to a new Nature study, ritual human sacrifices may have helped build and sustain social hierarchies.

Archaeological evidence suggests that human sacrifice – which the researchers define as deliberate and ritualized killing to please or placate supernatural beings – occurred in African, American, Arab, Austronesian, Chinese, Germanic, Inuit, Turkic, and Japanese cultures. According to what’s called the "social control hypothesis," human sacrifice legitimizes class-based power distinctions by giving a supernatural justification for the taking of a life – the ultimate authority. In that case, social stratification might be one of the earliest forms of leadership, and it's what led to kingdoms, monarchies, and governments. 

To test this hypothesis, a team led by University of Auckland’s Joseph Watts turned to phylogenetic methods, often used to study evolutionary relationships among different species. This allowed them to examine the common ancestry of cultures, test for coevolution, and draw conclusions based on the order of the traits that evolved. 

The team focused on 93 traditional Austronesian cultures spanning a range of social structures: from small, egalitarian, kin-based societies to complex, politically organized units. The practice of human sacrifice was widespread throughout traditional Austronesian societies, which originated in Taiwan before spreading west to Madagascar, east to Rapa Nui, and south to New Zealand. The breach of taboo, the funeral of an important chief, and the consecration of a new house or boat were common occasions for human sacrifices.

The victim typically held a low social status (such as a slave), and the instigator held a high social status (such as a priest or a chief). As the authors described, "the methods of sacrifice included burning, drowning, strangulation, bludgeoning, burial, being crushed under a newly built canoe, being cut to pieces, as well as being rolled off the roof of a house and then decapitated."

For each culture, the team recorded the amount of social stratification and marked down the presence or absence of human sacrifice. Then they developed models to test the coevolution of human sacrifice and social hierarchy using linguistic evidence. 

Human sacrifice was found in 40 of the 93 cultures sampled: 5 of the 20 egalitarian societies (25 percent), 17 of the 46 moderately stratified societies (37 percent), and 18 of the 27 highly stratified societies (67 percent).

Their findings suggest that the practice of human sacrifice stabilized social stratification, legitimizing political authority and social class systems. It increased the chances that high social stratification would arise, and it prevented the loss of social stratification once it had evolved. "Unpalatable as it might be," the team wrote, "our results suggest that ritual killing helped humans transition from the small egalitarian groups of our ancestors, to the large stratified societies we live in today."