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 power play.
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 human sacrifice and how unequal or hierarchical a culture was.
"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, analysed historical data from 93 "Austronesian" cultures.
Human sacrifice was widespread in Austronesia: 40 of the cultures in the study practised some form of it.
Early Austronesian people are thought to have originated in Taiwan and 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.
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 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).
In the most egalitarian cultures, it was five out of 20.