Seems to me there are two sources of moral progress:
First, we have better information. Some our decisions about "right" and "wrong" are based on the outcomes of such actions, especially when compared to existing alternatives. Therefore, if we find a better alternative, if we discovering that we were wrong in predicting consequences of some action, or if the environment changes so that the same action will now have different consequences, we may update our perceptions of "right" and "wrong" in a way our ancestors would agree with (after being told what we know).
Second, our capacities for abstract reasoning are increasing, and we have a desire for a consistent morality. So if at some moment we realize that X and Y are just two examples of the same template, we will want a morality that says that X and Y are both "right", or both "wrong", or the difference must be explained by a difference between X and Y. Our ancestors would in general agree that morality needs to be consistent.
Today's post, Whither Moral Progress? was originally published on 16 July 2008. A summary (taken from the LW wiki):
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