I just finished a psychology course on the subject, and I have to say that I am on the fence about it. Do I think it causes issues in children? Honestly, no.
There are studies on so many parts of humans, but very very rarely do they take the larger picture into account. A study saying that spanking causes aggression, or lowered I.Q. can draw it's own conclusions. But do they take the twin and adopted studies of aggression in family members raised in different households, with different parenting styles into account? If I remember them right, they found that HOW a child was being raised didn't change the level of aggression seen between the siblings. What I took this to mean, was that aggression, depression, etc. was more a matter of the person's genetic material then their upbringing. Hopefully, being raised in a non-violent, upbeat atmosphere would lessen what was inherited, but that you can't change DNA even if you can change the environment.
I also think this might be why the data would be flawed. If the parents are raised with spanking, then they are aggressive, then the children are aggressive because of the spanking, not because of the DNA theory that has been studied showing that children with the same biology raised apart still share more characteristics of aggression and depression than children from a non-aggressive family DNA patern. Or that they get this from their parents in their DNA, not from any action that happened during the developmental years. The same can be said for I.Q.
Also, as part of the class, was a comment in the text that though sexually abused children were more likely to commit the same crime, most abusive parents didn't come from an abusive home. These were all in the Pearson published book on Introductory Psychology. I don't remember the exact statements without looking them up nor the edition, but I can look them up and give exact quotes if needed.
My point is, aggressive people need to learn less aggressive ways to deal with problems. They are not aggressive just because they were abused, spanked, etc, but because they have it passed on a genetic level from their parents. If banning spanking, and actually having parenting courses as mandatory as driving classes to help re-direct aggressive parents AND aggressive children would lower future acts of violence, then OK, I'm for it. But if, like in most cases of parenting, you are told that you should just know what to do, and what is wrong to do, etc when you inherited the same problem from your parents, then controlling your aggression with just an act of spanking, as a last resort, with other options exhausted should be allowed as long as it is without leaving marks. Though, I will admit that I think there is a fine line between a spanking and crossing the line into abuse.
So, like I said, I am on the fence.