Note that the psychopath (devoid of altruistic desires) can do wrong even though he has no reason to avoid doing so.
Yes, I very much agree with that. What I wasn't so sure about is what it means to say he "ought" not act wrongly. If the prescription is redundant, adding nothing new to the general value claim, then that's unproblematic. But if it's supposed to express some further fact, then I'm not sure what it is.
Our psychology is such that we are motivated to fulfill our (own) desires. This is how I understand prudential prescriptions as having "force". Motivational force is a perfectly naturalistic and comprehensible phenomenon. But the force of morality is less clear, since we are not intrinsically motivated to fulfill other people's desires.
If morality lacks intrinsic motivational force, what other "force" is left? The idea of some supernatural "Big Force" behind morality makes no sense to me (literally - I have no idea what is meant by such claims - does anyone else?). But I don't think it really matters if morality doesn't have prescriptive force in this sense. We have other reasons to care about and promote it, as I argue here.
So, relating this back to my rejection of analysis (1), I can understand (1) only if we interpret 'ought' in the weak sense of 'morally ought' that effectively reduces to the descriptive statement "X is morally right". But of course it would be empty/circular to then use this as an analysis of morality! I prefer to take (3) as fundamental, and interpret (redefine?) normativity in terms of this.
Jim makes another interesting point which I wanted to address:
The key is that if the man on the street means by normative force what the Big Force people do, you're sunk and naturalism fails. You would then have to choose full-blown nihilism about morality or accept Big Force non-naturalistic objective realism. I maintain that the man on the street agrees with me that motivation not to do wrong to the old lady boils down to your not desiring her to suffer harm. ("I would hate to see her crying over her lost wallet later on.") He doesn't think of Big Forces. ("I don't care about the old lady, I just don't like to do anything Wrong," is something he doesn't say.) Thus naturalism goes through and the Big Force objective realist is left out in the cold implausibly maintaining that you need Big Force to avoid full-blown nihilism about morality.
That sketch of the "man on the street" seems fairly plausible to me. But I'm not so sure it matters either way. Why should we care what ordinary language refers to? Reality remains the same no matter how we describe it. Suppose we stipulate that "morality" refers to the Big Force nonsense. I'm then a nihilist about morality. But that's fine, because I don't think that's what's important. Let's use the word "schmorality" to refer to what I mean by 'morality', i.e. analysis (3) discussed previously. I'm a realist about schmorality, and can give a naturalistic account of it. Everyone (collectively) has reason to care about and promote schmorality. This is the concept that matters. If ordinary language is talking about something else, then so much the worse for ordinary language.