Haven't seen this debate in at least a couple of days so if you're interested.....

I did spank my daughter when she was small. I was very consistent that I did the calm assertive thing, that I didn't spank out of anger or frustration. But, my calm assertive had to be backed with a threat. That threat was a spanking that was followed with a hug a few moments after. I stopped gradually around 7 and 8, when she was able to reason better. I did take away privileges. My daughter excelled and is doing well in college.

I heard smacking kids hands can... my son gets butt spanks every now and then... but only if he's done something really bad..

I disagree with the study. I saved spanking life threatening offenses. All mine are past the spanking age and are all happy, challenging themselves, completing goals, and never been in a physical fight.
I didn't plan of spanking my kids when I had my oldest, she was almost 5 when I told her to stand next to my car while I got my purse out and locked up before entering a restraunt. She chose not to listen to me and ran towards the door of the restraunt and almost got hit by a car. It was then that I realized that it would be better to spank her butt and make her listen than to let to get herself killed. Our whole lives got alot easier after that, lol.
I'm not saying it's the only way, but it worked for us.

I disagree with the study. I saved spanking life threatening offenses

I disagree also I spank my boys on the butt and on the butt only when it is the last resort and they aren't listening or if it is life threatening or puts them in harms way

I think there are a lot of ameliorating or mitigating factors in individual situations, so you will not always see the "worst" harm evidenced in every case of a spanked kid. Just like "not spanking" is not going to ensure unharmed kids. There are other ways to cause the same results (which is basically that a child introjects or accepts conditions of worth) that lead to the harm they observed. The degree or extent of that is going to determine damage, not whether or not it was accomplished via spanking.
I'm opposed to spanking because I believe it can cause lasting harm & because I believe it's not necessary. But I also avoid other common & accepted parenting practices because I believe they can cause harm, & I don't assume they are necessary components of discipline. So it's not that I'm "anti-spanking" in particular. I think any behavioral focus relies on principles that don't promote optimal health or development.

I also recognize that the kind of harm or damage they are talking about (such as the "internalizing kinds" of difficulties) occurs on a spectrum of human experience, much of which is considered "normal" or "expected" just because we take it for granted!
It may be common but this doesn't mean it is normal, necessary or inevitable.
There's also a long timetable for these things. If there's a lot of repression going on in childhood (which often is the case with "good" or adaptive boys & girls who respond "well" to punitive discipline), the actual cost may not be apparent until much later. Nor is it always tied to its cause, even when it does present.
Therapists' client loads include people who are very successful, very competent (high-achieving, law-abiding, Harvard-graduates, bright, CEOs, etc.) & widely considered "well-adjusted" & happy. This is thought-provoking when it comes to assuming that success confirms "lack of harm."

I love my child too much to lay a hand on him.