A Review of "To Spank or not to Spank", which includes a good summary of the main points in the book, and while not being in favor of spanking, offers some practical advice on when it works and when it doesn't (when in anger, when it humiliates the kid, when it's too long after the facts, when the kid doesn't understand why ...).
A detailed summary of research on corporal punishment, combining the insights of researchers both in favor of and against a ban on spanking. A few quotes:
Child perceptions of the parent's wishes matter, as some misbehavior may be due to lack of parental clarity, and some due to child processes. Thus, future research likely should distinguish among kinds of misbehavior.
Child attributions about the parent's actions likely matter too. Attributions that the parent acts in the child's best interest and supports the child are likely to spell different consequences for physical punishment than attributions of hostility and anger alone.
[O]ne study that showed that adults, after having children, generally decreased their opinion of the effectiveness using corporal punishment.
While it may be that corporal punishment leads to increased aggression, it is also possible that increased child aggression leads to increased corporal punishment (as some studies have shown) or that some third factor causes both parental use of corporal punishment and childhood aggression.It seems to me that a lot of arguments against spanking don't apply to "properly applied" spanking (not in anger, understood by the child, etc.), and some gain most of their strength by rhetorical tricks such as talking of "solving problems by violence". Still, one very believable problem is moral internalization:
... this is a general problem with external rewards and punishments: they tend to replace internal motivations. A child punished when, for example, he failed to be polite to his grandmother, may be less likely to want to be nice to his grandmother, and may instead only do so out of fear of punishment.
- Moral Internalization - She found that CP decreases internalization of moral rules. This is concerning in that parents are more likely to use corporal punishment when they believe the child is at fault for some misbehavior. Thus, using a method that decreases moral internalization to respond to a failure to adhere to internal rules the child should have known is likely to perpetuate the problem.
The other downsides may be due to the problems the authors mention - studies not sufficiently distinguishing between spanking and abuse (partly due to abusive parents reporting it as "normal" discipline), the difficulty of distinguishing cause and effect, etc. Still, spanking itself may still have some negative effects.
Finally, there's a review of an article on Parenting Types, which classifies parents as low or high on control, and low or high on warmth, and summarizes how children of those parents tend to defer. The best results seem to be for "Authoritative" parents (high control, high warmth). The profile of the children of authoritarian parents (high control, low warmth) fits what I wrote above about internalization of moral values:
Their children have a multitude of problems, and are less individuated and show lower internalization of pro-social values, ego development, and perform more poorly on cognitive tests and see their parents as more restrictive.