What cemented my vague desire not to smack the sproglet was the one time that I did.

I was soooooo fucking irritated by her throwing food off her high chair tray a few months ago that I slapped her little hand (she would have been about 15 months), and the next day she did the same thing again, watching me with the weirdest expression on her face, then as soon as she'd done it, she slapped her
own hand. I felt like such a piece of shit.
She'll never be slapped again, and I have to say that when she plays with other kids her own age (I take her to quite a lot of playgroups and stuff), it's VERY noticeable that the kids who get their bums swatted regularly by their parents are the same ones that go around slapping and hitting the other kids.