War is both underdetermined and overdetermined. That is, many conditions are sufficient for war to occur, but none are necessary. Some societies remain peaceful even when significant risk factors are present, such as high population density, resource scarcity, and economic and ethnic divisions between people. Conversely, other societies fight in the absence of these conditions. What theory can account for this complex pattern of social behavior? ... [Margaret Mead] dismissed the notion that war is the inevitable consequence of our "basic, competitive, aggressive, warring human nature." This theory is contradicted, she noted, by the simple fact that not all societies wage war.