The amity-enmity complex is a term which was introduced by Sir Arthur Keith. His work, A New Theory of Human Evolution (1948), posited that humans evolved as differing races, tribes, and cultures, exhibiting patriotism, morality, leadership and nationalism. Those who belong are part of the in-group, and tolerated; all others are classed as out-group, and subject to hostility: "The code of enmity is a necessary part of the machinery of evolution. He who feels generous towards his enemy... has given up his place in the turmoil of evolutionary competition."
Conscience in humans evolved a duality: to protect and save friends, and also to hate and fight enemies. Keith's work summarized earlier opinions on human tribalism by Charles Darwin, Alfred Russel Wallace, and Herbert Spencer.