I would wager that judicious use of physical punishment is not considered in these “studies”.
Spanking has been seen as effective for millennia.
I suspect that they are using a different definition of “a single positive benefit” (isn’t that redundant?)
They could not find a single case where a child was spanked and the spanking reduced the number of times the child engaged in the bad behavior?
It strains my credulity.
Parents do not like to spank. Necessary discipline.
If they do, it isnt discipline. It is over the line.