Just ran across this thread, and, going back to the broad topic, figured that I'd point out that there are some academic psychologists who have done a lot of research and writing about about this very question, with some intriguing theories.
For instance, you might look at papers by Jonathan Haidt ( website: haidt.socialpsychology.org ) , as several are freely available. A recent one that directly addresses this topic is "Social intuitionists answer six questions about moral psychology." The paper is a bit academic-y, but is interesting and based on some neat research. BTW, their answer to where we get our morals from, which is merely the overview, is:
Very interesting site, sopsyched; and not a digression at all.
In brief, in Haidt's social intuitionism, moral thinking is a chain with six links:
1) Intuitive Judgment -- gut reaction: preconscious feeling an act is either right or wrong.
2) Post-Hoc Reasoning -- search for arguments to support one's Intuitive Judgment.
3) Reasoned Persuasion -- use of arguments to reach a moral consensus with others.
4) Social Persuasion -- urge to conform to group consensus, reasoned or not.
5) Reasoned Judgment -- unhappy with Intuitive Judgment, using logic to override it; produces "dual attitude", public vs private
6) Private Reflection -- imaginative sympathy triggers new Intuitive Judgment, original or contradicting original
Haidt etal. argue that the last two links are very rare, in cases of genuine moral dilemma; most moral thinking is covered by the first four. As #1 & #4 are preconscious, and #2 & #3 stem from #1, this model doesn't allow for much impartial, "objective" reasoning, traditionally the sine qua non of morality. Which is why they cite Hume, "reason is slave to the passions", as a philosophical basis.
My intuition (heh) is that cognitive intuitionism is correct, we aren't blank slates, but I'm not sure about moral intuitionism; there are certainly behavioral urges, unchosen gut feelings which we strongly identify with and rationalize, but I don't know if they're "moral" until we decide they are; urges need to be filtered through experience and group stucture before they're moral... maybe; if it's true that intuitions, independent of reason, are moral, then it forms a sort of middle ground between moral objectivism and relativism; the forms of moral intuitions, "fairness", "concern", "right", "duty", "reputation", "identity", etc, would be universal to humans, part of any individual or group morality; while the content, though tending toward an evolutionary optimum, "helping is better than harming" or somesuch, roughly "objectivism", would of course be subject to individual mutation, sometimes propagated through the group, hence "relativism".
Anyway, lots to think about. Thanks again for the link.