If, as a psychologically 'normal' human being, I find acts that I would call 'immoral' to be repugnant (and my gut response to immorality 'feels' different enough form aversion to, say, eating excrement, so that I can be justified in calling only one of those 'immoral') than I already have a motivation for acting morally: it 'feels wrong'.
However, what would a moral realist, who believes that the definition of ethical behavior is independent of human feelings of aversion, say about motivation for acting morally? Even if a moral realist has a perfect definition of morality, how do those definitions provide a motivation for humans to act morally? According to moral realists, does the motivation for acting morally even have to be related to an objective definition of morality?
I see no connection between standard forms of moral realism and motivation for ethical behavior:
Divine Command Theory: besides for the obvious ontological problem of whether such a thing exists as a 'divine command', this doesn't create moral obligation. If a god told me "if you act morally then you'll get to eat pie in Heaven, but if you don't, you will burn in Hell for eternity", than of course I'd be motivated to act morally, but only because I like pie and dislike fire, not because such acts are moral.
A Categorical Imperative as a Free Agent: while Kant, if I understood him correctly, believes that acts can only be moral if they come out of conviction that such acts are morally obligatory, I fail to understand why I would want to do those things that are obligatory.
Naturalism: the way that I've seen this view presented by Nicholas Sturgeon (among others), 'moral' is a quality, but not one that includes anything that could be constructed as a reason to act in that particular manner (though there's been a lot of discussion on this, here for example, I haven't seen anything compelling)
Pragmatism or Reciprocity: there's an idea that I should be motivated to act morally towards others, since I don't want people to take advantage of or harm me, if they were to find out that I acted immorally to them they would feel justified in doing so. However, this practical motivation still seems like a far cry from any moral imperative. If I can be sure that I won't be caught, I have no reason not to harm others.
Am I missing something? Is there such a thing as a purely moral motivation, or one based purely on a feeling of obligation towards moral principles that doesn't appeal to human feeling? Put differently, is there any way to compel a psychopath to act morally when no one is watching?