Corporal punishment does not actually administer a direct cause-and-effect link between a behaviour and its bad consequences. It is just as likely that it will create an unwanted association with the presence of the one administering the punishment (. the parents) as the bad consequence itself or its cause, contributing to further rebellious or sneaky behaviour, making its effect temporary. Education and nonaggressive, impersonal deterrents (such as educating them about the financial and psychological consequences of teen pregnancy) are far more effective if you want to show them a direct cause-and-effect relationship.
In the UK, spanking or smacking is legal, but it must not cause an injury amounting to Actual Bodily Harm (a "serious" injury such as visible bruising, breaking of the whole skin etc.); in Scotland since October 2003 it has been illegal to use any implement or chastise to the head when disciplining a child. In England and Wales slippering (. striking the buttocks repeatedly with the sole of a slipper) has been the traditional method of punishing children in the home. It is assumed still to be legal, provided that no long-lasting bruises are caused.