On the issue of nature v nurture (sorry to take this thread on a tangent), I do believe that children are born with a certain temperament but environment can shape it for better or worse, particularly during the first 3 years when the brain is still forming connections. And yes, it is possible for the environment to be different in the same family because of gender, birth order, parental perceptions/prejudices etc. There is support for this in studies, though the debate will always rage on.
But apart from temperament, there is also the relationship between the child and parent that over time, will trigger certain reactions. For example, I am super stubborn and strong-willed (wonder where my dd gets it from), and I am generally sullen with my mother, occasionally descending into screaming at her. She used to hit me - that is my most lucid memory of my childhood relationship with her. No doubt, my mother thinks I am still the difficult child she brought up.
With my dh, I am actually quite reasonable and don't shout- (oh well, not more than a handful of times over the years. That is because I feel I am listened to and he does not smack me!
Anyway, in a long winded way, what I am trying to say is that the potential damage to the relationship between an already strongwilled child and his/her parent is the most worrying thing about physical punishment.