Survival of the Pig(ish)

Dawkins bases much of his theory on evolutionary psychology, the field of study that tries to explain psychological traits from an evolutionary perspective. A common theory in evolutionary psychology is that, in prehistoric times, people lived in a fierce, competitive, life-or-death situation, where they had to fight tooth and nail against each other in order to gain access to resources necessary for their survival.

Therefore, by behaving selfishly, they increased their chances of surviving and passing on their genes.