He actually didn't use scientific studies to refute that argument, just poor logic. A lot of that guy's arguments were really weak, and the way he talked made me want to hit him harder than Animeniax wants to hit his kids.
Regardless, whether low IQ scores and Corporal Punishment have a causal relationship or merely a correlative one, it's still in the kid's best interest not to use violence as a negative punishment strategy, as evidenced by 1 minute of researching the issue online and Y's articles posted earlier in the thread.
I'd also like to say that as weak as anecdotal evidence is, I see the effects of corporal punishment on a day to day basis, and as someone who is in the business of child behavior and management strategies, resorting to physical punishment is completely ineffective in the face of a mountain of other resources.
Hell, even if it were stacked against other negative punishments it still does a pretty terrible job. The evidence supporting its effectiveness is so inconclusive, and the evidence of its negative effects so numerous that it's silly to even consider it. It's a way to vent frustration, nothing more.
Edit to below: I was just on my way to correct that. Thank you. My ideas got jumbled.