"The biological necessity for morality arises because, for the species to survive, any animal must have, on the one hand some egoism -a strong urge to get food for himself and to defend his means of livelihood; also- extending egoism from the individual to the family to fight for the interests of his mate and young. On the other hand, social life is impossible unless the pursuit of self-interest is mitigated by respect and compassion for others. A society of unmitigated egoists would knock itself to pieces; a perfectly altruistic individual would soon starve. There is a conflict between contrary tendencies, each of which is necessary to existence, and there must be a set of rules to reconcile them. Moreover, there must be some mechanism to make an individual keep the rules when they conflict with his immediate advantage."
Joan Robinson Economic Philosophy