This seems a difficult line of reasoning. First, the idea of "one's interest" is ill-defined. Absent elucidation it reduces to some small set of innate or acquired desires (longevity, reproductive success, etc). If that is what is meant, then it seems to me that the claim about the "overwhelming majority" is quite possibly wrong. While people do work to further such interests, they often sacrifice or overlook such interests in pursuit of their vision of "ought". And this seems to be true across the spectrum, from left-wing humanists to right-wingers. And it also seems true in trivial day to day behaviour where we adhere to codes that inconvenience us and in no way advance our interests (again seen in a narrow, *computable* sense).
Second, the above seems to suggest that I should discard my dishonesty (which I tend to think, no doubt self-servingly ;-), as morally motivated solidarity) and say, if applicable, hey I have a good shot at becoming a functioning capitalist or at least serve my interests better by fucking over the workers who have no shot at similar escape, and I should bloody well take it.
The response I anticipate to this inference of mine is that I am ignorant of my own status as a worker and someone who is getting fucked over myself. Let us pass lightly over the patronising nature of such a comment (and I mention it only because you are responding to Michael's characterisation of morality-denial as "arrogant").
But such a response (to my point) is no different from saying that there is a different, broader framework, both of analysis, and of defining "interests", by which the prescription is justified and appropriate. And that, among others, is the argument for morality as well. Amartya Sen makes a similar point, in a slightly different context, in his amusing and short paper title "Rational Fools: A Critique of the Behavioral Foundations of Economic Theory".
--ravi
-- Anyone who takes an effort to intellectually challenge the status quo and established habits is infinitely more venerable than hacks defending that status quo and established habits, regardless of the truth function of their propositions. -- W.Sokolowski