I'm interested that a 63 year old would be of the 'generation that definitely smacked'. My Mum's almost sixty, I was born in '78 and AP theories were really catching on by then. Weren't the seventies quite a gentle time to be raising children?
Anyway, I wasn't smacked, I won't smack. But I find spidookly's posts on this really interesting; we do, as adults, use our physical size to discipline children even if we don't inflict pain. I have to hold my toddler down to effect nappy changes, or administer medicine, and that feels awful to me and is probably physically uncomfortable for her, but it's done lovingly. Sometimes it's not done quite as lovingly; during the worst tantrums, I've had to use a fair amount of strength to get her into her car seat, and although I don't intend to hurt her, I'm still bringing superior force to bear to get her to do something she doesn't want to do, you know?
So is the difference the pain? A short smack, of the sort some people have described, is going to fade very quickly. Is the difference because the pain is the point, not a side effect? But all discipline is aimed at causing discomfort to the child, physical or emotional, yes?
I don't know. I know I won't smack, and currently I don't use any 'punishment' like time-out or whatever, but I do think it's an odd balance. The reason it's different from a husband hitting a wife is, I guess, because the nature of parenting means that you have to do physical things against their wills when they're little, so that dynamic is already set up and to a lot of people it seems like an extension of that, to smack.
I won't intentionally cause my child pain, though, it just comes down to that, to me. And also, I guess, there's an element of wanting something in reserve. I only shout if there's something massively important she needs to hear - like, don't run onto a road - because if I shouted a lot, she wouldn't get the distinction between 'that was naughty' and 'that was life threatening'. Same with smacking. If they still don't listen, where do you have to go from there?