First and foremost, this article isn't about science. The key aspect about the scientific method is being able to reproduce results, not collect statistics. Any subject matter pertaining to human beings involves far too much variance in too many variables. Science can NEVER prove that hitting your kids is bad, nor can it EVER prove that not hitting your kids is good. At best this psuedo-science can show that there is a statistical correlation between hitting your kids and general negative effects in the long run. But even then the sample sizes, cultural settings, and religious affiliations have to be taken into account. It cannot ever be precise.
That being said, I can only speak from experience, not generalities. I found that in my own childhood, it was the fear of getting hit that stabilized my behavior as a child, rather than actually getting hit. The times I did actually get hit, it wasn't that bad, but it was my parents' disappointment in me that made me want to change my behavior. If the author of this article is so eager to convince me that hitting my kids isn't going to benifit them in the long run, why didn't he present alternative solutions? Am I supposed to just wait until my kids turn 18 and miraculously mature on their own?
I have 12 first cousins, all with children, and I've seen situations where non-hitting punishments work, and those where they don't work. Children are not to be reasoned with, nor are they to be negotiated with, at least before puberty. The most successful disciplining techniques I have witnessed are those that are centered around fear and respect of the parents. These are not mutually exclusive, nor are they automatically paired.
The most important point is that parents' should never hit their children out of anger. If you do discipline your kids by hitting, it should be very routine: the same room, the same time of day, the same force, and the parent should be calm throughout the entire process. I wonder how many people in the "hitting your kids" study confomed to these principles?