Anyway, not to retread old debates, but I recently came across a bit of Kant in which he address the charge Posner lays. I can't resist sharing, although it may already be familiar to some readers. In the Grounding for the Metaphysics of Morals, sec. II, Kant gives his view of the issue in footnote 25:
I have a letter from the excellent Sulzer in which he asks me why it is that moral instruction accomplishes so little, even though it contains so much that is convincing to reason. My answer was delayed so that I might make it complete. But it is just that the teachers themselves have not purified their concepts: since they try to do too well by looking everywhere for motives for being good, they spoil the medicine by trying to make it really strong. For the most ordinary observation shows that when a righteous act is represented as being done with a steadfast soul and sundered from all view to any advantage in this or another world, and even under the greatest temptations of need or allurement, it far surpasses and eclipses any similar action that was in the least affected by any extraenous incentive; it elevates the soul and inspires the wish to be able to act in this way. Even moderately young children feel this impression, and duties should never be represented to them in any other way.So there you have it; the reason for the historical sterility of ethical theory--in the Posnerian / Sulzerian sense of "it makes no visible practical difference"--is that philosophers overcomplicate matters. All you have to do, Kant thinks, is show a person what they already regard to be the most praiseworthy sort of act, namely, an act done from a motive of duty.
(Philosophers of education might incidentally note that Kant also purports to have insight into the moral psychology of children, and to recommend a certain sort of moral education.)