I think there's some evolutionary pressure on morality, so that it's a mixture of requirements for behavior which improve the odds of survival for the group, maintain the status of high-status people, and/or are just people making things up because they sound cool/seemed like a good idea at the time/distinguish the group from other groups. People are encouraged to think of all three as the same sort of thing.
My thinking is along similar lines... yet I divide the three modes from a slightly different perspective. I don't put much weight on the 'odds of survival for the group' for example, although there is definitely a sense in which moral claims are meant to be interpreted as declarations of pro-social norms. I had tended to leave off 'just making things up because they sound cool' because I was focusing on the explicit political influences but come to think of it "seemed like a good idea at the time" probably does explain a lot!
[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.