Personally, I've clearly seen the very bad effects of corporal punishment on children. It saddens me that there isn't currently a budding body of research about this, amongst educators and child psychologists.

I've worked with small groups of kids in many different settings. And what I've seen is that regular spanking of small children can cause a couple different outcomes. The child can become hyperactive - in other words, the child becomes squirrely, because this behavior is a way to get the parent in a better mood, and avoid the slap for awhile.

I've also seen elementary school kids who become bullies of their peers in school, and the neighbor kids - because that's what they're learning from their unwitting parent. The kids are learning that you ought to impose morality on others using physical agression.

Sweden outlawed spanking or smacking of kids in 1979. The law is very well written, in that it doesn't provide for any punishment of perpetrators. However, if what the parent did to the child can be defined as assault, there are laws which come into effect there.

Here's a great explanation about the Swedish policy by Adrienne Haeuser