Perceived loss of purpose or meaning may discourage belief in moral realism
Consider this short biographical account of the much lauded proponent of moral realism, Derek Parfit.
There was a single philosophical question that began to preoccupy him. Was morality objective?
Parfit was desperate to believe that it was. But many philosophers believed that it was not, at least not in the sense that Parfit meant.
The brief article goes on to describe how Williams believed that it doesn’t make sense to speak of facts about what we have reason to do independent of what we want to do. This allegedly brought Parfit “to the edge of despair.”
Parfit believes we have reasons that are independent of our desires to abstain from, e.g., punching people because they irritate us.
He grew increasingly troubled by philosophical disagreement. He came to believe that dissent about ethics—especially dissent between leading philosophers—was evidence for the relativism of ethics. And he thought relativism essentially collapsed into nihilism. If your moral truth conflicted with, but was no less valid than, my moral truth, then this showed that, ultimately, nothing mattered.
I suspect this concern with nihilism results in an asymmetric bias against antirealist positions. In particular, from the vantage of those like Parfit, a world without stance-independent value may seem bleak, meaningless, and awful. It is the worst thing that could possibly be true, because it would preclude any and all possibility of anything being genuinely good or truly mattering or actually being worthwhile, and so on.
The consequences of realism being true, if one is an antirealist, seem to me to be much less catastrophic. Motivated reasoning undoubtedly influences both realists and antirealists about morality or value more generally. But perhaps in at least one important respect those disposed towards moral realism experience a greater degree of motivated reasoning.