Lawrence H. Keeley at the University of Illinois writes in "War Before Civilization" that 87% of tribal societies were at war more than once per year, and that 65% of them were fighting continuously. He writes that the attrition rate of numerous close-quarter clashes, which characterize endemic warfare, produces casualty rates of up to 60%, compared to 1% of the combatants as is typical in modern warfare. "Primitive Warfare" of these small groups or tribes was driven by the basic need for sustenance and violent competition.