When I am working with parents and this subject comes up, I always site Dr. Bruce Perry's brain research with his anecdotal description of what happens when a child is spanked.
Dr. Perry calls it a brain bath or in other words it's like being under water at a pool, you can hear people talking on the pool deck, but you cannot understand what they are saying. So if you, the parent, are trying to 'teach your child a lesson' by spanking them, literally they cannot learn with corporal punishment.
Because the minute a child feels that painful stimulus they release stress hormones, adrenaline and cortisol, bringing on the brain bath and the child in turn only wants to avoid the source of that pain (the parent) and or resent the parent who is administering the pain. This in turn confuses the child because the object of their great affection (attachment) is also someone to fear and someone whose touch they cannot trust.
This informational approach always seems to give parents an 'ah-ha' moment. More than not parents then say to me, "That is so logical, now please tell me what to do instead"...and this becomes a jumping off point for parent education. As you can see this is a subject near and dear to my heart as I have worked as a trainer for the CA Department of Child Abuse Prevention and Neglect for many years.
Thoughtfully submitted, Ann Corwin
Ann Corwin, Ph.D., M.Ed.