Kraco has it right. In the right child-rearing environment, spanking is not something resorted to easily or with regularity. I can think of maybe 3 times in my childhood where we did stuff bad enough to get spanked.

The studies you all are referencing are most likely referring to something closer to child abuse. Normal disciplining of your child through spanking won't hurt the IQ, unless maybe if you strike them in the head. I suppose it could hurt them emotionally/psychologically enough so they perform poorer on IQ tests, but that link is still dubious.