Most of us would agree that human sacrifice is a bad idea. Yet many ancient civilizations (and some more modern ones) engaged in religious rituals that involved sacrificing people. Why do so many societies evolve a system of human sacrifice, despite the obvious moral drawbacks? A group of social scientists has just published a statistical analysis in Nature that reveals how this grisly practice has fairly predictable results, which benefit elites in socially stratified cultures.

The group examined 93 Austronesian cultures in the Pacific Islands, drawing information from the Pulotu Database of Pacific Religions to determine which groups had human sacrifice and when. Previous analysts have suggested that human sacrifice helps to maintain social stratification. In this new study, the researchers wanted to understand the relationship between human sacrifice and social stratification over time.

To do that, they created statistical models using Bayesian methods, testing to see how human sacrifice affected societies that fit into three buckets: egalitarian, moderately stratified, and highly stratified. They write:

Evidence of human sacrifice was observed in 40 of the 93 cultures sampled (43%). Human sacrifice was practiced in 5 of the 20 egalitarian societies (25%), 17 of the 46 moderately stratified societies (37%), and 18 of the 27 highly stratified societies (67%) sampled.

The researchers ran these societies through several different probabilistic models, exploring how the cultures had changed over time and what role (if any) human sacrifice played in those changes.

What they found is probably not too surprising, though it is revealing. Human sacrifice has the effect of maintaining stability in highly stratified cultures, and it can also turn a moderately stratified society into a highly stratified one. Interestingly, egalitarian societies that introduced human sacrifice did not become stratified.

Human sacrifice, in other words, is a useful tool for elites who want to maintain their power in a stratified society. This is especially true in the Austronesian context, where religious and political leaders were often the ones doing the sacrificing, and the sacrificial humans were generally slaves or people with low social standing (see figure below for some of their calculations).

It's worth noting that this finding reflects religious human sacrifice in Austronesian cultures—which are themselves fairly diverse—and may not be relevant to cultures where humans are sacrificed by the state or during racial/ethnic genocide. It's also important to keep in mind that it's extremely hard to quantify human sacrifice and social status, so these findings represent only the broadest and most abstracted trends.

Still, it's a finding worth pondering. As the researchers conclude in their paper:

Unpalatable as it might be, 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.

They also point out that their work calls into question recent research on the pro-social role of religion in the development of civilization. "Religious rituals also played a darker role in the evolution of modern complex societies," they write. If the "ritual" you're talking about is human sacrifice, that seems like a fair assessment.

Nature, 2016. DOI: 10.1038/nature17159