Most animals rarely kill their own. Among non-human primates, for example, many species live in social groups and fight with their neighbours. Battles involve loud calls, displays, chases and occasional grapples – just enough for the larger or stronger group to convince its opponent to retreat. Yet although inter-group contests may be intense and frequent, and although they may determine access to critical resources, they hardly ever lead to death. For most primates the goal is to win or to retreat without being damaged.

But humans often aim to kill. So why is our species different? Why do we regularly and deliberately kill our own kind?

At the simplest level there are two kinds of answer: killing is either unnatural or natural. The contrasting positions are encapsulated by the ideas of Jean-Jacques Rousseau and Thomas Hobbes, who have become icons for opposing views of human nature. For Rousseau, we are an essentially peaceful species. For Hobbes, war is our natural state. As we will see, both positions can be criticized.

Rousseau's thinking was dominated by the concept of the ‘noble savage’. He was so impressed by reports of people's spontaneous tendency to care for each other that he claimed conflict was absent in a state of nature. ‘With passions so tame’, he wrote, ‘and so salutary a curb, men, rather wild than wicked, and more attentive to guard against mischief than to do any to other animals, were not exposed to any dangerous dissensions’.