My oldest has Aspergers Syndrome and was EXTREMELY oppositional as a child. He would do the opposite of everything. He would resist everything. It was more important for him to fight the battle, than to get his own way.
Spanking him turned out to be a mistake because it only made him more determined to fight back. I still tried it, though, many times. But, like Kevin said, you cannot do it in anger, and I am sure that's the biggest mistake I made.
That son grew to hit back and he and his father even came to blows. Very bad .
However, now we have a good relationship and the really trying times of the past are long forgotten.
The younger child needed only the smallest of swats to be completely chastised. I think he was spanked maybe three times . Totally different person.