Man by Nature is based upon the ongoing brain research that is providing new insight into how our brain functions. It shows how the left-brain Interpreter function makes it possible for us to be instinct-driven tribal territorial animals - consistently quarreling and warring with one another - and yet to be oblivious of the instincts provoking our behavior. The book identifies ten traits characteristic of human tribes, and develops a "Tribal Programming Theory of Human Behavior" that explains most observed human behavior remarkably well: It makes the case that human tribes are based primarily upon shared tribal beliefs that are defended as fiercely and irrationally as territory, thus making it almost impossible for religious and political tribes (for example) to discuss issues rationally. It concludes by weighing what will be required for the world and its tribes to eventually live in peace, given the new knowledge of our human nature.