Less effective in what setting? If physical punishment is more common world-wide, that is rather telling evidence that our forebears found it to be the most effective way to bring children in line, and it has been done ever since; at least until western civilization decided it was improper or at the very least, sub-optimal.

My point is that who is the AAP to say how best to raise children? I seriously doubt the validity of any study that claims to be conclusive on how to raise children, and would make a bet that any research they did was localized to the US. Anecdotally, several of my friends were beaten as children and turned out fine. All this proves is that sometimes physical punishment works and sometimes it doesn't. But the same goes for "establishing a loving relationship with your child;" since you can hardly do that when they're threatening to stab you with a knife.