Studies can show anything you want them to show, it depends on how they were done. If it is subjective data........take any study with a grain of salt. Personally, I don't buy into those types of studies as often they're rigged to prove a point.
Nor do I see spanking as a "corporal punishment" either. It is a method, used wisely, that can be extremely effective on some children. As with any method, though, used too often and the wrong way can produce no results or actually worsen the issues.
My mom used spanking for everything, I do mean everything.......and most of the time I wouldn't call it spanking either. When we were young it was extremely effective..........I mean no little kid wants a belt across their bare rear multiple times. As we approached pre-teen years........it was so over used that it no longer phased us. We'd take it in stride and do as we pleased regardless.......while taking greater care not to get caught. Usually by age 12.......mom gave up. Because she used no other real method of discipline.....after that it was mostly empty threats.
This is why I opted out of spanking for the most part. Instead I chose other more creative methods to deter unwanted behavior. I also leaned heavily on making the consequence suit the offense. Example: after a certain age if you broke something that wasn't yours you were required to earn the money to replace the item. If I thought one form of discipline was becoming ineffective due to over use....I slipped another into it's place. Spanking was a last resort or a means to quickly grab their attention or to let them understand how serious something they did was......such as the safety issues. By not over using it or relying on it heavily, on the rare occasion I did spank.....it most certainly got their attention and had the desired effect.
Now, I will say that I am huge in explaining to a child what they did wrong and why it was wrong too. If you never do this you might as well throw up your hands and just walk away. It drives me insane to watch parents just tell their kids No or Stop It or whatever or just up and punish them and yet they never actually tell the kid what they did wrong. How is the child suppose to know?? Osmosis? And yes, I've had parents tell me their kid is "just supposed to know" what they did was wrong whether they're a toddler or a teen. Or they assume because they told them once, or twice, or ten times that it registered and they remember minutes, days, weeks, months or however long later. omg Children have short memory when it comes to this stuff or they're testing boundaries....if you don't point out those boundaries when they overstep, they don't think it is there. Yes, it gets repetitive and monotonous as all get out for the parent (especially if you have more than one child) but that is just part of parenthood.
My mom never explained what or why something was wrong. We were left to guess. If we guessed incorrectly......we got the belt. (we got the belt for everything minor or major) Since her rules were somewhat subject to change with her state of mind.......we guessed wrong quite often.
When I state that my mom abused me (and my sibs ) it's not necessarily the spankings (or rather beatings) that I refer to......although many did fit the bill, not all did. I certainly didn't walk away from it scarred for life. Sorry but the emotional / mental abuse was far far worse, thanks. Sibs say the same........and they're still trying to shake off those years of emotional/mental abuse.
I've met many a parent who will readily condemn anyone who lays a hand on their child, but who is so verbally, mentally abusive to their children that it is deeply disturbing, yet the parent can't see it because "I never laid a hand on them." Words can be just as bad if not worse.
The method doesn't create the abusive environment. It's the way the parent uses it. I knew a woman who used the suggested means of discipline on her daughter (I babysat her). She used time out and grounding. Great huh? Uh, not so much. Time out could last hours and grounding usually was for several months to her room. The little girl was FIVE. Mom had no clue of normal child development.......and also was often punishing her severely for things a typical 5 yr old will do when playing with other children. Mom wasn't trying to be abusive. Mom just had no real parenting skills and no one to use as a good example to base her own parenting on. So I spent a year educating her on child development and the proper way to discipline (with age appropriate methods).
I personally think child development should be a required course in both Jr High and High School. I say Jr High because so many very young girls are sexually active at younger and younger ages.
Sooo, yeah. It's usually not the method, but how it is used that pushes it over the line into abuse.