There is an american sociologist, Murray Straus, who’s spent 40 years studying the effects of smacking.
His study of 800 children shows that kids who were physically disciplined regularly had an IQ 5 points less than the non-spanked kids.
Unfortunately that suggests to me there must have been a lot of kids being beat regularly in my home town.
Some links on the topic:
http://www.breakingnewsenglish.com/0909/090928-iq.html
http://www.dailytelegraph.com.au/lifestyle/spanking-long-used-by-parents-to-discipline-naughty-children-can/story-e6frf00i-1225779278575
http://www.huffingtonpost.com/2009/09/25/spanking-kids-iq-lower-fo_n_300006.html
And a video.