I'd say many people decide that moral/ethical behavior is best for their own self-interest.
Well, I would agree. But, on a tangential point, if something is in one's self-interest, can it be described as moral/ethical?
I don't know, but if you're interested, here are my thoughts. Professionals have codes of "ethics". Most circumscribe behavior that would be in the individual's self-interest (at least short-term) for the good of society. Almost all professionals must act AGAINST their own self-interest at times in order to comply with ethical standards. Contrast that with attorneys. Their code of "ethics", for example, states that all defendants deserve a defense. Therefore, it is "ethical" to defend everyone. But that just gives attorneys cover to act in their own self-interest (having a job) and is not ethical by anyone's dispassionate definition. Sorry for the irrelevant rant.