In the following excerpt Irwin describes Joyce's argument:
The philosopher Michael Ruse has argued that “morality is a collective illusion foisted upon us by our genes.” If that’s true, why have our genes played such a trick on us? One possible answer can be found in the work of another philosopher Richard Joyce, who has argued that this “illusion” — the belief in objective morality — evolved to provide a bulwark against weakness of the human will. So a claim like “stealing is morally wrong” is not true, because such beliefs have an evolutionary basis but no metaphysical basis.This is a kind of philosophical make-believe, an attempt to live "as-if" there were objective moral duties because it's very hard to live without them. Even so, if one didn't believe was stealing is wrong why act as if it is? What's the point of trying to conform to what you "know" isn't true? Irwin wonders about this, too:
But let’s assume we want to avoid the consequences of weakness of will that would cause us to act imprudently. In that case, Joyce makes an ingenious proposal: moral fictionalism.
Following a fictionalist account of morality, would mean that we would accept moral statements like “stealing is wrong” while not believing they are true. As a result, we would act as if it were true that “stealing is wrong,” but when pushed to give our answer to the theoretical, philosophical question of whether “stealing is wrong,” we would say no.
The appeal of moral fictionalism is clear. It is supposed to help us overcome weakness of will and even take away the anxiety of choice, making decisions easier.
There is, though, a practical objection to moral fictionalism. Once we become aware that moral judgments have no objective basis in metaphysical reality, how can they function effectively? We are likely to recall that morality is a fiction whenever we are in a situation in which we would prefer not to follow what morality dictates. If I am a moral fictionalist who really wants to steal your pen, the only thing that will stop me is prudence, not a fictional moral belief.But if a philosopher is willing to consign God, freedom, and morality to the realm of fiction, why place a value on truth? Why not go all the way and just say that all "truth" is really just fiction? The fictionalist can give no answer to this question.
It is not clear that this practical objection can be overcome, but even if it could, moral fictionalism would still be disingenuous, encouraging us to turn a blind eye to what we really believe. It may not be the most pernicious kind of self-deception, but it is self-deception nonetheless, a fact that will bother anyone who places value on truth.
Irwin goes on to describe how Joyce defends his moral fictionalist account and how he applies it to God and free will. I encourage you to check out the article at the link.
One of the tests of the soundness of one's view of the world is whether or not it's actually possible to live consistently with it. If a worldview entails that one must live as if fictions are true or truth is fiction perhaps there's something wrong with the worldview. Evidently, Joyce has adopted a worldview that requires him to embrace "fictions" like the existence of God, free will, and morality even though he believes these things don't really exist. He holds to a worldview, in other words, with which he can't really live. Indeed, he finds himself living as if his worldview is false. That's a good sign that it probably is.