I wrote my best shot at a conclusion because, despite a recent Huffington Post piece calling the "debate" settled, the research on spanking is messy and controversial. The same study might be cited by different experts as evidence that spanking is safe and that it's dangerous. That's in part because the effects of spanking are hard to evaluate: You can't assess its effects like you would a drug, in a randomized controlled clinical trial, because it's unethical for researchers to instruct a random group of parents to spank their kids and a second group not to. Yet in the name of science, that type of study would be ideal, because it would allow scientists to compare what happens to both groups of kids -- and conclude, with some certainty, that any differences arising between them were due to the spanking.
But for physical punishment, the best researchers can do is compare what happens to kids who are punished with those who are not. And the thing is, kids who incite spankings may be more difficult or delinquent to begin with than kids who don't. Ideally, studies should control for a child's behavior prior to spanking, to ensure that findings only reveal how many more problems kids develop after they are spanked -- but researchers don't always do this, in part because they often collect information about people at a single point in time. Researchers might ask adults if they were hit as kids and, at the same time, assess these adults for mental illnesses, looking for associations between the two -- such as in this 1999 study. (The recent study on harsh punishment was conducted in this manner, too.) But when these types of studies report that kids who were spanked have more behavioral problems later in life than kids who were not, it's impossible to pin the cause on spanking.