Ethicist James Rachels examined the nature of social instincts and envisioned a three-step process of morality, based on reciprocal behavior:
- Kin altruism, showing special regard for one’s kin
- Group altruism, extending altruism to the larger group of which one is a member
- Widespread altruism where each and every member of the human race is held worthy of regard.
Morality is, therefore, "reciprocal altruism, " which we can see is a notion identical to the Golden Rule.
This non-diety based morality could certainly explain why non-believers can be moral without any reference to God, and develop an entire system of ethics that have no reference to theology.