Well, I got 2 of the 4 spankings in my life because I was engaging in dangerous behavior and did not listen...then I hurt myself.
After my mother bandaged my heavily bleeding wound she spanked my bottom. (the second spanking in this context was actually unfair, as she removed the bandage the gauze was stuck to the wound and it hurt, so I cried....)
I was maybe 4 at the time. (Oh, I was playing with a rug making tool that had a razor blade in it...sliced the top of my left middle finger nail off - wanna see?

- it's healed but I have the hint of a scar under the nail)
There is a difference between spanking and beating IMHO. At an age when reasoning does not work, a pop on the bottom seems to be an effective way to redirect the attention.
At an older age...I guess we can argue about that.
Personally, I much prefer spanking over 'time out' , which to me is close to psychological abuse.
Then again, not every child responds the same way.
One kid is fine with time-out, not phased at all, another is in fits.
I guess the main problem is, how is the punishment administered. In anger and excessive? Or pop and done?
I find it interesting to read the results of this study, but I wonder how they picked their test subjects and how do you do a 'double blind' in a case like this.
Do the non spankers really never raise a hand, are the spankers really this evil?