A study recently released by the University of New Hampshire and The prevention Research Center in California shows that children punished by spanking and hand slapping or other non-cognitive methods were likely to have IQ's that were lower by 5-20pts(Stanford-Benet testing). They did several studys using different groups of 2-9yo children and came up with the same findings. Here's the link, there's a lot more to it.
http://pubpages.unh.edu/~mas2/CP51.pdf
http://pubpages.unh.edu/~mas2/CP51.pdf