A plausible evolutionary hypothesis suggests itself: limited resources led to the selection for within-group cooperation and between-group competition in humans, resulting in within-group amity and between-group enmity.

• 合理的演化假说告诉我们：有限资源所造成的选汰力量，会促进人类群体内的合作与群体间的竞争，因此导致群体之内的友好与群体之间的敌对。