Annual War Deaths in Small-Scale versus State Societies Scale with Population Size Rather than Violence

In The Better Angels of Our Nature: Why Violence has Declined, psychologist Steven Pinker cites mean ratios of war (battle) deaths suffered annually per 100,000 individuals as evidence for concluding that people who live in states are less violent than those who live or lived in “hunting, gathering, and horticultural societies in which our species spent most of its evolutionary history.” Because such ratios are blind to actual population sizes, it remains to be seen whether the apparent decrease in contemporary violence is an artifact of scaling factors. Here scaling of war deaths is quantified relative to actual population sizes for 11 chimpanzee communities, 24 human nonstates, and 19 and 22 countries that fought in World War I and World War II, respectively. Mean annual battle deaths expressed as percentages of population sizes scale inversely with population sizes in chimpanzees and humans, indicating increased vulnerability rather than increased violence in smaller populations. However, the absolute number of mean annual war deaths increases exponentially (superlinearly) and nearly identically with population sizes across human groups but not chimpanzees. These findings suggest that people evolved to be more violent than chimpanzees and that humans from nonstates are neither more nor less violent than those from states.