The myth is that behaviors that adults demonstrate as adults are encoded in their children as a statement of authority–ie, to spank your child affirms your authoritative position over them. This is halfway true, meaning it does exemplify authority, but not specifically to you, the parents. What it means is that the child learns that physical aggression is one way to express authority, and thus, they will be more inclined to use it themselves in situations when they want to dominate someone else (another kid, for example) or reclaim a sense of authority if they feel threatened (so, for example, when a kid hits their teacher). This also goes for other types of extreme behaviors (yelling/shouting, for example).
I teach JrH/HS, but my colleagues who teach ES are staunchily against spanking because as teachers, what they see is that parents who spank their kids (and yes, there are nuances to how you define ‘spanking’ and the circumstances/frequency with which it’s done), aren’t actually disciplining them, but simply punishing them. Discipline is to teach your child what’s right and what’s wrong and why. Spanking doesn’t actually do that. What it does do is tell the child to do things (or not do them) to avoid pain and fear, rather than to actually change their behavior. And that’s pain and fear coming from ONLY M&D. So, a child who throws toys at their parents when frustrated and gets spanked will learn very quickly not to do that at home in the presence of M&D, but at school, where teachers can’t exercise the same discipline, they will still engage in the behavior–hence why so many teacher friends of mine think the practice is bupkus.
Spanking in general is one of those things that is either a net-neutral OR detrimental. Never is it positive in terms of raising the child. Given that its highest possibility is to be zero-sum, then the question becomes, why engage in a behavior that is potentially harmful when there are no positive benefits to it, especially if there are more effective ways to discipline a child?
Most of the time, defenders of spanking were people who were spanked themselves and feel that they have grown up to be competent, reasonable, and well-adjusted adults–and that is probably true. I don’t think that spanking has the power to override all of the other parenting choices that one makes, and certainly an involved, loving parent who uses spanking as a form of discipline is unlikely to “ruin” their child as a result of that choice. But, I have to quibble with the fact that such defenders usually seem to believe that spanking is necessary and submit that perhaps they only think so because it was how they were raised, which gives the appearance of necessity. My child is only 3, but I have so far yet to see an occasion in which spanking him has ever been necessary–safety issues and all–and I’ve never heard of a single justification from any parent.
I’m by no means one of those “Gen X” parents who feels that I should get down on my knees and have a touchy-feely discussion about parental rules and my choices when it comes to discipline, but spanking just makes no sense to me. There is a difference between effective and productive. Subjecting your child physical pain may be effective; it’s never productive, though.