Researchers have studied the grisly practice of ritual human sacrifice in nearly 100 cultures - including pre-European New Zealand - and found it was widely used as a tool to maintain power.
Researchers from the University of Auckland's School of Psychology, the Max Planck Institute for the Science of Human History in Germany and Victoria University, wanted to test the link between how unequal or hierarchical a culture was and human sacrifice.
"Religion has traditionally been seen as a key driver of morality and co-operation, but our study finds religious rituals also had a more sinister role in the evolution of modern societies," said study lead author Joseph Watts.
The study, published today in leading journal Nature, used computational methods derived from evolutionary biology to analyse historical data from 93 "Austronesian" cultures.
The practice of human sacrifice was widespread throughout Austronesia: 40 out of 93 cultures included in the study practised some form of ritualistic human killing.
Early Austronesian people are thought to have originated in Taiwan and, as they moved south, eventually settled almost half the globe, spreading as far as Madagascar, east to Rapa Nui (Easter Island) and south to the Pacific Islands and New Zealand.
Rulers, such as priests and chiefs, were often believed to be descended from gods and ritual human sacrifice was the ultimate demonstration of their power.
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Methods of ritual human sacrifice in these cultures included burning, drowning, strangulation, bludgeoning, burial, being cut to pieces, crushed beneath a newly-built canoe or being rolled off the roof of a house and decapitated.
Victims were typically of low social status, such as slaves, while instigators were usually of high social status, such as priests and chiefs.
The study divided the 93 different cultures into three main groups of high, moderate or low social stratification.
It found cultures with the highest level of stratification were most likely to practise human sacrifice (67 per cent, or 18 out of 27).
Of cultures with moderate stratification, 37 per cent used human sacrifice (17 out of 46) and the most egalitarian societies were least likely to practise human sacrifice (25 per cent or five out of 20).
"By using human sacrifice to punish taboo violations, demoralise the underclass and instil fear of social elites, power elites were able to maintain and build social control," Mr Watts said.
Professor Russell Gray, a co-author of the study, notes that "human sacrifice provided a particularly effective means of social control because it provided a supernatural justification for punishment.
"Rulers, such as priests and chiefs, were often believed to be descended from gods and ritual human sacrifice was the ultimate demonstration of their power."
A unique feature of the research was that the use of computational evolutionary methods enabled the team to reconstruct the sequence of changes in human sacrifice and social status over the course of Pacific history.
This allowed the team to test whether sacrifice preceded or followed changes in social status.
"What we found was that sacrifice was the driving force, making societies more likely to adopt high social status and less likely to revert to egalitarian social structure," study co-author Associate Professor Quentin Atkinson said.