Many people believe that a good old smack never hurt anyone and actually gets the message across, but there are others who think that hitting a child will either escalate into child abuse, or it doesn't do that, then turn the child itself into a criminal. The truth is somewhere in between. Research has shown that corporal punishment is ultimately self-defeating, but the absence of it alone doesn't guarantee a crime-free society.
Many parents will admit to having smacked their children when they felt there was no alternative. Some people will say that it works, others that they just felt so bad themselves that they would never do it again.
Punishment is the presentation of an unpleasant stimulus after a response. It makes the response less likely to occur in the future. To stop a behavior, punishment should be given right away and given every time. It should serve be severe enough to be worth avoiding, but not so severe to prolonged that it causes other undesirable behavior.