While a certain part of our moral tendency can be explained by an innate concern for our fellow beings, there also seems to be a human tendency to internalize the values of those around us, so that even though we might not expect to get caught for certain transgressions, we still feel bad about it. Similarly, even though we don’t expect any material gain for sacrificing our lives for a cause, some of us will still do it. This isn’t what I would normally call “self-interest”.
On the other hand, I think morality often serves the useful purpose of advancing self-interest by advancing the interests of the group. If I had a choice of cultures to join, all other things being equal, I would prefer to join one in which people help each other out and make occasional sacrifices for the group benefit than to belong to one that was strictly dog-eat-dog. This would be true even though I was required to internalize the group moral system and would run the risk of having to make serious sacrifices myself. The reason I would prefer such a group is simply my own self-interest - the average happiness of people in a group that cooperates is going to greater than in a group that does not.
When moral principles from different groups are in conflict it makes sense to me to support the principle that most benefits humanity as a whole, since this serves our overall self-interest. Arbitrary sexual taboos, dietary restrictions, or required rituals are the sorts of principles that there is no point in promoting.


