I found the study quoted. It does not say what he thinks it says.
http://www. berkeley.edu/news/media/releases/2001/08/24_spank.html
Take the spaces out.
It's from 2001. 100 families who spank moderately. Specifically excludes frequent spanking. Does not include spanking with any objects. Only followed the families unto the children were 14 years old which in my opinion is the weakest point of all.