I lean strongly toward unitary theory, with two caveats. First, not all specific moral statements need be true or false; some can have the middle truth-value (or no truth-value if you prefer to say that). Second, unitary-ness is not a logical truth - if true, it's a consequence of the attitudes people actually have and the circumstances in which we find ourselves.
Why is there only one particular morality? Because people keep insisting on talking about it. We keep finding that, like Churchill, we prefer "jaw, jaw" to "war, war". We keep trying to justify our ways to each other and getting meaningful responses back, in which people try and generally succeed to find similar motivations in each other with which to assess proposed rules, virtues, and goods. We can usually tell the difference between a moral argument, offered in the spirit of reasoning together about what to do, versus an exercise in the dark arts of persuasion. And enough of us prefer the former to perpetuate the search for mutually agreeable goals, rules, and virtues.
What is the one true morality? It's too early to tell. But why think that there is one? Because of the great commonalities between human beings. Granted, we have different preferences, but in many cases it seems possible to step back from them and find a general principle like "I get to use my skills to create more of what I want, and you get to use yours to create more of what you want" - a principle that abstracts from individual differences, in a fair way, and leads to an outcome acceptable to both parties. In some cases it may not be possible - which is why I allow that some moral statements may be neither true nor false. (A theory should be no more precise than its subject matter.)
Where does morality come from? From our natures as both rational beings who communicate, and social beings who need to cooperate and derive enormous utility from that. This also answers the "why care" question - without implying what you call absolutism.
Mainstream philosophy comparison: Habermas and Scanlon hold roughly the same views on the points I've mentioned here.
This seems to me to be an ad populum fallacy. Just because everyone acts as if there is one, true morality doesn't make it so. Or am I missing something?
The great commonalities between humans might make many of our moralities very similar, but I think there are some differences. For one example, I think eating (factory farmed) meat is a great moral wrong, but I don't think there's anything, even in principle, I could say to convince some other people to share my view.
My meta-ethics are basically that of Luke's Pluralistic Moral Reductionism. (UPDATE: Elaborated in my Meta-ethics FAQ.)
However, I was curious as to whether this "Pluralistic Moral Reductionism" counts as moral realism or anti-realism. Luke's essay says it depends on what I mean by "moral realism". I see moral realism as broken down into three separate axes:
There's success theory, the part that I accept, which states that moral statements like "murder is wrong" do successfully refer to something real (in this case, a particular moral standard, like utilitarianism -- "murder is wrong" refers to "murder does not maximize happiness").
There's unitary theory, which I reject, that states there is only one "true" moral standard rather than hundreds of possible ones.
And then there's absolutism theory, which I reject, that states that the one true morality is rationally binding.
I don't know how many moral realists are on LessWrong, but I have a few questions for people who accept moral realism, especially unitary theory or absolutism theory. These are "generally seeking understanding and opposing points of view" kind of questions, not stumper questions designed to disprove or anything. While I'm doing some more reading on the topic, if you're into moral realism, you could help me out by sharing your perspective.
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Why is there only one particular morality?
This goes right to the core of unitary theory -- that there is only one true theory of morality. But I must admit I'm dumbfounded at how any one particular theory of morality could be "the one true one", except in so far as someone personally chooses that theory over others based on preferences and desires.
So why is there only one particular morality? And what is the one true theory of morality? What makes this theory the one true one rather than others? How do we know there is only one particular theory? What's inadequate about all the other candidates?
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Where does morality come from?
This gets me a bit more background knowledge, but what is the ontology of morality? Some concepts of moral realism have an idea of a "moral realm", while others reject this as needlessly queer and spooky. But essentially, what is grounding morality? Are moral facts contingent; could morality have been different? Is it possible to make it different in the future?
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Why should we care about (your) morality?
I see rationality as talking about what best satisfies your pre-existing desires. But it's entirely possible that morality isn't desirable by someone at all. While I hope that society is prepared to coerce them into moral behavior (either through social or legal force), I don't think that their immoral behavior is necessarily irrational. And on some accounts, morality is independent of desire but still has rational force.
How does morality get it's ability to be rationally binding? If the very definition of "rationality" includes being moral, is that mere wordplay? Why should we accept this definition of rationality and not a different one?
I look forward to engaging in diologue with some moral realists. Same with moral anti-realists, I guess. After all, if moral realism is true, I want to know.