Well first of all LOL at me asking you for information on your sources saying spanking a child is bad and your answer pretty much being "you show me one that shows the advantages" lolol
Second of all you are confusing discipline with brutality, it's not the same to swat a kid in the butt to distress them for a couple of minutes than it is to leave welts on their body, many states allow corporal punishment as long as it is not cruel and bizarre punishment and as long as it does not leave any marks, I dont fully agree with it because hitting a kid in the head, pulling ears, pinches, hair pulling and slaps across the face dont leave marks but they usually get out of hand and are quite de-moralizing to children.
Third of all I'm non religious and I believe spanking when used correctly can be an appropriate discipline technique.
As a last thought... I don't have children, I don't know what it's like, I have no idea how I would react to disciplining my own child, therefore I don't consider it fair to criticize those who are parents dealing with their children the only/best way they know/can.