Moral facts as facts about cooperation
I find the perspective of Sterelny and Fraser (2017) rather appealing:
while there is no full vindication of morality, no seamless reduction of normative facts to natural facts, nevertheless one important strand in the evolutionary history of moral thinking does support reductive naturalism—moral facts are facts about cooperation, and the conditions and practices that support or undermine it.
For moral thinking has evolved in part in response to these facts and to track these facts. So one function of moral thinking is to track a class of facts about human social environments, just as folk psychological thinking has in part evolved to track cognitive facts about human decision-making.
The idea that connects moral thinking to the expansion of cooperation in the human lineage has two complementary aspects. First, it is important to an individual to be chosen as a partner by others; access to the profits of cooperation often depends on partner choice. Choice, in turn, is often dependent on being of good repute, and (often) the most reliable way of having a good reputation is to deserve it. It is worth being good to seem good. Recognizing and internalizing moral norms is typically individually beneficial through its payoff in reputation. Second, human social life long ago crossed a complexity threshold, and once it did so, problems of coordination, division of labour, access to property and products and rights and responsibilities in family organization could no longer be solved on the fly, or settled on a case-by-case basis by individual interactions. Default patterns of interaction became wired in as social expectations and then norms, as individuals came to take decisions and make plans on the assumption that those defaults would be respected, treating them as stable backgrounds; naturally resenting unpleasant surprises when faced by deviations from these expectations. The positive benefits of successful coordination with others, and the costs of violating other’s expectations, gave individuals an incentive to internalize and conform to these defaults.
These gradually emerging regularities of social interaction and cooperation were not arbitrary: they reflected (no doubt imperfectly) the circumstances in which human societies worked well, and how individuals acted effectively in these societies to mutual benefit. Given the benefits of cooperation in human social worlds, we have been selected to recognize and respond to these facts. So this adaptationist perspective on moral cognition suggests that normative thought and normative institutions are a response to selection in the hominin lineage for capacities that make stable, long-term, and spatially extended forms of cooperation and collaboration possible.
No doubt there are trade-offs between the size of the cooperation profit and its distribution. But despite these complications, a natural notion of moral truth emerges from the idea that normative thought has evolved to mediate stable cooperation. The ideal norms are robust decision heuristics, in that they satisfice over a wide range of agent choice points, typically providing the agent with a decent outcome, in part by giving others incentives to continue to treat the agent as a social partner in good standing. The moral truths specify maxims that are members of near-optimal normative packages—sets of norms that if adopted, would help generate high levels of appropriately distributed, and hence stable, cooperation profits.
So no adaptationist, truth-tracking conception of the evolution of moral thinking will deliver a full, clean vindication of diverse moral opinion. Indeed, we expect the moral case to be intermediate in a variety of respects: First, our moral practices are a mosaic; some elements may turn out to be vindicated, others revised, others discarded. Second, as we have noted, moral judgements function to signal, to bond, and to shape, not just to track; vindication is only in question with respect to tracking. Third, as we shall now explain, tracking is only partially successful; moreover, its success may well have varied across time and circumstance.