An anonymous commenter found this fantastic article on a blog in the United Kingdom.
Question: There is a 13-month old child trying to pull the network cable out of my laptop as I write this. Should I hit him? How hard?
Answer No 1: A light two-finger tap on the baby's hand together with sternly saying, "No" has a small chance of getting the message across to a 1 year old, though it is probable that the baby's brain is not yet ready to make the connection,
Answer No 2: The baby's brain most certainly is ready to make such a connection (the stern NO would probably do on it's own) - the trouble is that in most cases the parent won't be able to provide the necessary consistency.
Pain is an absolutely essential learning tool for children *including* babies. When a baby get his first teeth he will bite his tongue, it will hurt, he'll do it again, it will hurt again, and he'll quickly associate the two and learn not to bite his tongue. The same applies throughout early childhood - we evolved to feel pain for a reason - and young children *cannot* be taught not to damage their bodies in other ways. If anyone is stupid enough to think they can - try googling for the "Congenital Insensitivity to Pain" and read the stories of children with this disease. Or watch "A Life Without Pain". Most have their teeth pulled to stop then chewing their tongue to bits. Most end up in wheelchairs by the time they are teenagers, if they are lucky.
I'm sure if parents of these children had a button they could press to cause their child pain every time they bit their tongue, or ran into a tree, they'd do it, and probably be accused of violence by the loony anti-smacking brigade.
The causing of the child pain isn't the problem with smacking, since as per above it's an essential learning tool. The problems with smacking are consistency and association. In order to work, the child must be associate the pain with the "bad action", and the punishment must be consistent (it must happen every time the "bad action" occurs). This is very hard to achieve - but exactly the same problem applies to *any* system of punishment or reward.