Collective violence (war, terrorism, violent political conflicts, genocide, repression, organized criminal activity, disappearances, torture, and a range of other abuses of human rights) killed at least 200 million persons directly or indirectly, and injured many more, during the 20th century (Krug, Dahlberg, Mercy, Zwi, & Lozano, 2002). Recent advances in the science of behaviour can help to explain why humans turn so quickly to collective violent actions in response to threat or opportunity, and why such actions are in a real sense “natural.” However, in the contemporary world the potential costs of relying on these nearly automatic responses are unacceptably high. This paper explores the roots of this violence from the perspective of the natural science of behaviour. Strategies for modifying collective violence should take into consideration motivational factors such as verbal processes and cultural perceptions, shifting motivating antecedents, etc. Rules, models, and structural conditions are also rel...