I would even argue that if you're only acting a certain way to avoid pain then you're not really a better person than you were before, just self-serving.
I respect your position, Pie, but I'm still struggling to see how this issue wouldn't arise in the case of verbal discipline or taking away of privileges as well. Nobody in the thread seems to have a problem with disciplinary measures as such- as I pointed out in my last post, some form of a reward-punishment structure can be incorporated even in a very productive moral education system (very few of our actions, even as adults, are done only from a sense of moral duty without having some sort of ulterior motive somewhere in the subconscious. It's only natural an education system of morality would take that basic fact into account). Given that fact, can you really say there's an in principle objection to be found against physical disciplinary measures that's also not available against verbal ones?
I continue to feel the line that's being drawn here to exclude physically disciplining a child under any circumstance is rather arbitrary, in the sense that the arguments used here can very well apply to verbal disciplinary measures as well. I don't see how moral concerns crop up the moment we cross over from the realm of verbal to physical measures. Rather, I think the line should be drawn at inappropriate behavior towards children in general. When judging whether an action crosses a moral red line, we shouldn't ask whether that action is verbal or physical, but the effect it would have on the child, the intent with which the action was carried out, and a lot of other circumstantial factors.
I'm by no means a cultural relativist, I'm as full-blooded a moral realist as they come. However, the reason I think Angie's (and others') appeal to culture is not altogether irrelevant to this conversation is because, again, there's no a priori moral line that could be drawn to begin with. There's no moral fact of the matter which precludes physical disciplinary measures without also discounting verbal ones (or vice versa, for that matter). There are instead considerations of appropriateness and effectiveness, which vary from situation to situation, and of course from culture to culture. It's only because of the fact that our shared morality fails to adequately shed light on the issue that cultural norms become relevant.