Did human sacrifice create social stratification?

New research has found that the more socially stratified a culture, the more likely ritual homicides were carried out in its history.

Historical records suggest that, thousands of years ago, many societies carried out ritual homicides to appease their gods. What historians still don't understand, however, is why.

"How could something so costly and devastating have been so common in early human societies?" asks Joseph Watts, a doctoral student studying cultural evolution at the University of Auckland in New Zealand.

Some researchers have hypothesized that ritual killings helped reinforce social hierarchies, contributing to the evolution of social classes that still exist today. A new study digs into this hypothesis to see if there is a link between the stratification of a society and whether such killings were part of its history.

"We found strong evidence that human sacrifice coevolved with social stratification and that it helped to build and maintain it," Mr. Watts, who led the new study, tells The Christian Science Monitor in a phone interview.

The new study, published Monday in the journal Nature, focuses on 93 traditional Austronesian societies, cultures associated with a family of languages that originated in Taiwan. 

Watts and his team sorted the societies into three categories: egalitarian, moderately stratified, and highly stratified. Those communities in which individuals did not inherit wealth or status were coded as egalitarian, those in which individuals inherited wealth and status distinctions but could rise or sink in status in their lifetime were moderately stratified, and those that had inherited strong wealth and social status distinctions that were unlikely to change over a generation were highly stratified. 

Of the highly stratified societies, 67 percent had evidence of human sacrifice in their history. Moderately stratified societies had 37 percent and egalitarian ones had 25 percent.

The team applied that data and coding to a family tree of sorts mapping out the evolutionary relationships between these communities to see if there was a causal link.

"What we found is that human sacrifice stabilizes social stratification in general," Watts says. The practice likely reinforced a class system, and may have even promoted such social stratification.

"There was generally a power difference between the perpetrator and the victim," Watt explains. "It was generally those of high social status that initiated the ritual and those of low social status, such as slaves or captives, that became the victims." 

"In these cultures there was a massive overlap between political and religious authority," Watt says. This meant that religion could be exploited as a tool for social control.

And it was likely a good way for the populace to accept grisly practices like human sacrifice, says Davíd Carrasco, a historian at the Harvard Divinity School in Cambridge, Mass. and author of the book "City of Sacrifice," who was not part of this study but who's own work has found a similar link. With supernatural authority behind the killings, it might seem justifiable, he explains in a phone interview with the Monitor. 

These ritual, religiously motivated homicides took many forms. They could be rites related to food sources, a major natural disaster, a death of a ruler, punishment for violations of social rules, or other events.

It makes sense that the more stratified societies would have been more likely to practice such ritual killings for another reason, Jan Bremmer, a historian at the University of Groningen in the Netherlands and editor of the book "The Strange World of Human Sacrifice," tells the Monitor in a phone interview. Communities with class systems were usually much larger than egalitarian ones. 

"If you have only a small community," he says, "You're not going to destroy valuable human resources."

Dr. Bremmer agrees that a correlation between a history of human sacrifice and the stratification of a society exists. But he isn't so sure about causality. 

The new study used a binary system, simply marking a society as having evidence of such ritual killings in the past or not. It overlooks the specifics of how frequently, on what occasions, and how such religiously motivated homicides were used, Bremmer says.

And Bremmer says those details could help paint a better picture of how human sacrifice influenced societal evolution. "We should first look these different functions before we start to say this promotes a stratified society," he says.