Most theories of human nature (especially those that inform people's politics) boil down to two simplistic notions:
Human beings are basically bad. People are intrinsically selfish, violent, impulsive, and destructive. Their natural state is evil. Only by virtue of some external constraint (by the social system, by the law, by God) can people be pulled out of their natural evil into a state of good.
Human beings are basically good. People are naturally cooperative, loving, empathetic and altruistic. Left to their own devices, people are "noble savages." Only the corrupting forces of bad parents, bad societies, and bad states can thwart people's inherent goodness.
Generally speaking, conservatives believe people are naturally bad, and liberals think people are naturally good. Conservatives focus their attention on controlling and directing the individual with rules in order to preserve the common good (e.g. "law and order" campaigns, big defense budgets, etc.). Liberals focus their attention on controlling collective organizations (governments, businesses, families) in order to preserve the individual good (e.g. government mandates for worker safety, "safety nets," etc.)
I think most people are moderates, since they find the first position too cynical and the second too naïve. The truth is that human beings are naturally selfish and naturally cooperative. Things would be a whole lot easier if it was one way or the other; then we wouldn't have to spend so much time trying to figure out which response was the right one.
This poses some problems for anyone trying to construct a consistent model of fairness . . . since fairness is only consistent with about half of human nature.