Smacking children makes them less intelligent

A researcher who has "spent 40 years studying the effects of corporal punishment" has concluded, according to the Daily Mail, that smacking children makes them less intelligent, and that the more children are smacked, the less intelligent they become.

The full Daily Mail article is here:
http://www.dailymail.co.uk/sciencetech/article-1215985/Smacking-makes-child-intelligent.html

The argument seems to be as follows:

• Smacked children are less intelligent than those who are not smacked.
• The more often a child is smacked, the less intelligent s/he is likely to be.
• Therefore smacking your child makes the child less intelligent.

 
On the other hand, this could simply be an example of correlation not proving causation.  It may well be that children who are smacked tend to be less intelligent than those who are not smacked, but there are other ways in which this could be explained.
 
It is possible that parents who smack tend to be less intelligent than those who do not.  If so, their children are likely to be less intelligent too, since intelligence is strongly hereditary.

A less kind explanation is that the children who get smacked could be the less intelligent children.  Maybe they are getting smacked because they have done stupid things.  Maybe there is a connection between low intelligence and bad behaviour.