The justification for corporal punishment is the same as every other savage, nonsensical, or barbaric tradition our culture continues. Legacy. Tradition. Religious belief. My grandparents did it, so it must be right. Some angelic being appeared in a fiery shrub screaming "neek!", and told a goat-herder it was true 3,000 years ago (according to this book of questionable origin), so it MUST be right!
If you are a homo sapien, your natural neurological response to pain is to shutdown the learning pathways in the emotional and social learning centers of the brain. Specifically, the amygdala directly triggers high response in the ventromedial prefrontal cortex, retarding high-order "social" logic and emotional cognitive function. PERIOD.
For anyone to argue that the biological function of the ventromedial prefrontal cortex differs significantly in males and females? You might as well argue that the earth is flat. Because that is 100% nonsense. The same response that occurs in males also occurs in females. This is also verifiable empirical fact, confirmed via fMRI and PET imaging.
Massive volumes of experimentation have taken place in this area of neurological research, due to its wide application in many various fields of medicine. Pain response is not one of the gray areas of neurology, it is a well-established field with strong research behind it. When you argue the facts, you are arguing literally thousands of experiments in which functional imaging has actually SHOWN these results.
If anyone wishes to argue those results, you are free to join in the debate. Just be aware that it will take more than your Bible, your personal anecdotal experience, or "my Gramma told me so" to enter that conversation and be taken seriously. Be prepared to show significant fMRI, PET, CAT, or other verifiable empirical evidence to disprove what has already been discovered.
You can start by arguing the results of this research (I'll wait for your response in this thread):
http://gc.nesda.com.br/Conteudo/Arquivo ... nitiva.pdf
Whether or not you choose to accept the FACT, every time you strike your child as punishment, you are harming their intellectual development - specifically in the context of social learning and emotional cognitive development. You are actually handicapping their ability to learn from their mistakes.
If you're okay with handicapping your child's cognitive abilities, be my guest. Just understand that you are holding them back from fully developing into who they might have become had they not had a barbaric monkey for a parent.