My best friend's daughter was the most defiant child I have ever seen! She was like this from 18 months on and my friend spanked her A LOT! I even remember being concerned about it. Well, that girl is now a teenager and the sweetest most caring and resposible teen out there! My son was the complete angel when he was little and I didn't need to spank him. Well, he's a teen now and I would use completely different adjectives to describe him from my friend's daughter!
There are so many things that play into the shaping of a child and spanking may or may not help. The bottom line is you need to be consistant, show love and form a real relationship with your child. (And even that's no guarantee during the teen years!)
