"I would love to see whether any psychological study has ever been done to confirm that systematic spanking does escalate to beating? "
I worked in domestic violence for several years and perused that literature up till a few years ago. There was never any evidence that spanking of children as a disciplinary tactic was related to child abuse or any other violence.
That doesn't mean that in any given case a spanking parent couldn't become abusive also, but it is not a risk factor.
Thanks for your comments.