"In most foraging societies, said Fry, lethal aggression was infrequent, and in the archaeological record violence didn’t take regular group-on-group character until relatively recently, when people settled down in ever-larger, more complex and hierarchical societies. In the new paper, Fry and Soderberg looked at ethnographic histories of 21 nomadic forager societies, compiling a database of every well-documented incidence of lethal aggression that could be found in reputable accounts spanning the last two centuries.
They counted 148 incidents in all, of which more than half involved a single person killing another. Only 22 percent involved multiple aggressors and multiple victims, and only one-third involved conflicts between groups."
This illustrates the kind of sloppiness of which I'm speaking.
Link: https://psycnet.apa.org/record/2007-05299-000