I know all about these researches and the 'experts' that earn good money producing them. I also know that if spanking was just half as harmful as claimed then it already many milleniums ago would have caused such disasters that humanity would have self-destructed. Hitting can be used in abusive ways and then it no longer is spanking but violence but you can hurt and harm a child much worse without ever spanking. What matters is that the child experience being loved, wanted and respected; if it does that it also instinctively understands and accepts that it must be punished for misbehaving. And spanking is a swift and easily understood kind of punishment - as opposed to most if not all kinds of non-physical punishemnts.
Obviously you did not read the study and how it defines spankings. You also misunderstand that just because something is harmful and increases likelihood of negative outcomes it does not mean the victim of the abuse will become a serial killer.
I know it and it is very nice theoretically. The problem is just that it does not describe how life in reality is and it does not take into consideration that words can be amd often are much more hurting and harmful than spanking.