Aggression between groups is determined partly by willingness to fight, which depends on a number of factors including numerical advantage, distance from home territories, how often the groups encounter each other, competitive abilities, differences in body size, and whose territory is being invaded. Also, an individual is more likely to become aggressive if other aggressive group members are nearby. One particular phenomenon – the formation of coordinated coalitions that raid neighbouring territories to kill conspecifics – has only been documented in two species in the animal kingdom: 'common' chimpanzees and humans.