We teach our children to solve problems with physical force almost from the time they are old enough to walk. We do this by using spanking as a form of discipline. We control them with either violence or threats of violence. Even parents who spank infrequently will use threats like You're going to get a spanking as a means of control. True, many adults who were spanked as children do not become violent.
People react to unpleasant experiences in many different ways.
The point is that spanking does not contribute to a positive self-image and in the long run does considerably more harm than good. I was in a store one day and witnessed a scene that I will remember all my life. It was a simple common occurrence, but it had an incredible influence on me. I saw a woman who appeared to be a victim of violence because she had bruises on her face. Perhaps she just fell down or ran into a door - but I doubt it.