I'm not surprised at the study. It's basically impossible to control for between individual variation sufficiently without an RCT, but that will never happen with spanking. There will always be this problem of which end to label the cart and which end to label the horse.

I guess I'll also poke the hornet's nest while I'm here....

I don't really have a problem with corporal punishment. Not for children, or adults, when appropriately administrated. I say this a person who has firsthand experience with a public school that utilised corporal punishment and an angry parent just taking it out on you for being a kid.

At school a paddling was just another step in the process. You'd lose recess time, you'd have to clap erasers or write lines, you'd get sent to the principal's office and then and only then get a paddling on you return trip if you kept it up. There always had to be two witnesses and the teacher who sent you down wasn't allowed to do it. If that still didn't work they would call your parents to come get you and paddle you again. The last one basically never happened

It was so different from someone who was angry at you just trying to make themselves feel better that I could easily recognize it even as a small child. It always baffles me when people deny any daylight between the two, I assume it's born out of a very fortunate ignorance. I never felt unsafe at school. It never diminished my trust in anyone there. If I got paddled at school—I knew it was coming, I knew exactly why, and I knew I could have made a sensible choice to avoid it. That was not the case at home.

The other reason is the moral one. I never see people with well behaved children claim you can forego punishments entirely and I have yet to hear a reasonable explanation for why intentional emotional harm has more virtue in it than intentional physical harm. Because that is what the alternatives are: isolation and deprivation. A time out isn't harmless, losing recreation and exercise time isn't harmless, they just don't leave marks you can see.