Though human sacrifice is a thing of the past, Watts and others believe that understanding what motivated it is still relevant because other manifestations of extreme inequality do persist—slavery, for example. If they could identify the purpose human sacrifice served, then perhaps they could propose more-humane ways of achieving that purpose, thereby making the world a better place.

The question of the social function of human sacrifice is not new, but until recently efforts to answer it have drawn purely on anecdotal evidence. For Harvey Whitehouse, an anthropologist at the University of Oxford, that approach is deeply flawed, because it allows researchers to cherry-pick the evidence that supports their pet theories. A better method, he says, is to pool data about large numbers of historical societies in databases, and test theories against them.

Watts and colleagues have built one such database, Pulotu, that stores information about more than 100 traditional Austronesian cultures. Whitehouse and others have developed Seshat—another database that covers more than 400 societies that existed across the globe over the last 10,000 years. “We’re trying to develop a whole new methodology that adjudicates more objectively between competing hypotheses,” says Whitehouse, “so that we end up with a more robust picture of the human past from which we can extract genuine lessons for the future.”

Over time, as societies became larger, they also tended to become less egalitarian and more hierarchical. In 2016, the Jena group reported that Pulotu data support the so-called social control theory, according to which human sacrifice stabilized societies as they became more stratified, by legitimating class distinctions and political authority. It is probably no coincidence, Watts says, that the victims were often people who posed a threat to the elites, or who had fallen out of favor with them.

The results coming out of Seshat—which have yet to be published—suggest that social control may not be the whole story, however. No society in Pulotu comprises more than a million people, while Seshat includes “mega-empires” whose subjects numbered in the tens of millions. Seshat’s founders therefore argue that it tracks social complexity closer to modern levels, and they find that, beyond around 100,000 people, human sacrifice becomes a destabilizing force. “Our suggestion is that this particularly pernicious form of inequality isn’t sustainable as societies get more complex,” says Whitehouse. “It disappears once they pass certain thresholds, because they cannot survive with that level of injustice.”

Rather than being an essential stepping stone to greater complexity, the Seshat team argues, at these thresholds human sacrifice became a parasitic practice—an attempt, often by military heroes who had transformed themselves into “god-kings,” to seize and maintain power, to the detriment of social cohesion. That’s because, whereas human sacrifice might have terrorized the members of a smaller, simpler society into obeying their self-styled leader, it could no longer do so in a large and ethnically diverse one. There, it was easier to disobey the ruler, or desert, and evade punishment—and the temptation to do so only grew stronger as societies grew larger.