Spanking is one of those hot button parenting topics and most people have a strong belief on one side or the other. When considering discipline methods, I always talk to parents about 1) what are you teaching your child 2) are you modeling the behavior you want your child to emulate and 3) what is the long term impact/consequence of the discipline method you are employing.
Discipline comes from the word disciple, which means "to teach". So, it's important to consider what your discipline methods are actually teaching your children. When you use spanking as a discipline method, you are sending the message that big people can hit little people and hitting is a way to resolve problems. If you spank your child when you are angry you are modeling that when you are angry it's okay to hit. If you are using spanking as a disciplinary method but then get upset with your child for hitting you, a friend or their sibling, they are getting a mixed message. Young children particularly are unable to discern why it is okay for you to "spank" but not okay for them to hit when they get upset or their sister isn't doing what she is supposed to be doing and they hit them. It sends mixed and confusing messages.
While spanking may get the immediate response you want, the long term consequences may not be what you are hoping for. The immediate payoff may not be worth the long term consequence. Spanking begins to instill fear in a child and it teaches a child to be fearful of their parent(s) rather then teaching them to consider their behaviors and take responsibility for their actions. If used frequently, spanking can begin to create a sense of revenge in a child where they seek ways to get back at you, so something you intend to use as discipline begins to create the very behavioral problems you are trying to discourage.
Spanking is a form of hitting and shows a lack of respect for another person. If you expect your child to respect you, it's important that you extend the same to them. I often hear parents say, "I was spanked as a child and I turned out okay". It's important to understand that we have a lot more information than we had even 5 years ago about the brain and how it works. When a child is in fear, they can not think rationally or make a good decision. Utilizing the information we have now to be better parents and raise healthier children is just like using the advances we've made in medicine and technology. Why would we use a more dangerous and less effective medication for treating cancer when we have newer and safer methods? The same is true for parenting. Research has shown repeatedly, the ineffectiveness of spanking as a discipline method. It can damage your relationship with your child, create fear, mistrust and ultimately resentment. The best and most effective parents are connected to their children. If your child is afraid of you, it makes it hard for them to feel connected to you.