"The same reasons - revenge, honor, territory and jealousy over women - that fueled deadly conflicts in the Amazon continue to drive violence in today's world," said Walker, lead author and assistant professor of anthropology in MU's College of Arts and Science. "Humans' evolutionary history of violent conflict among rival groups goes back to our primate ancestors. It takes a great deal of social training and institutional control to resist our instincts and solve disputes with words instead of weapons. Fortunately, people have developed ways to channel those instincts away from actual deadly conflict. For example, sports and video games often involve the same impulses to defeat a rival group."

Walker examined records of 1,145 violent deaths in 44 societies in the Amazon River basin of South America by reviewing 11 previous anthropological studies. He analyzed the deaths on a case-by-case basis to determine what cultural factors influenced the body counts. Internal raids among tribes with similar languages and cultures were found to be more frequent, but with fewer fatalities, when compared to the less frequent, but deadlier, external raids on tribes of different language groups.