The Roots of War

Only three types of creatures engage in warfare--humans, chimpanzees, and ants. Among humans, warfare is so ubiquitous and historically commonplace that we are often tempted to attribute it to some innate predisposition for slaughter--a gene, perhaps, manifested as a murderous hormone. The earliest archeological evidence of war is from 12,000 years ago, well before such innovations as capitalism and cities and at the very beginning of settled, agricultural life. Sweeping through recorded history, you can find a predilection for warfare among hunter-gatherers, herding and farming peoples, industrial and even post-industrial societies, democracies, and dictatorships. The good old pop-feminist explanation--testosterone--would seem, at first sight, to fit the facts.

But war is too complex and collective an activity to be accounted for by any warlike instinct lurking within the individual psyche. Battles, in which the violence occurs, are only one part of war, most of which consists of preparation for battle--training, the manufacture of weapons, the organization of supply lines, etc. There is no plausible instinct, for example, that could impel a man to leave home, cut his hair short, and drill for hours in tight formation.
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  thanks to also not found in nature