I haven't been through it myself, but my friend's son was very aggressive from the time he was about 18 months to well over 2 years old. So much so that she didn't want to take him to play group, and I really didn't enjoy being around him.

But now, at 3, he is a much nicer, sweeter kid. He apparently is still quite "alpha" in personality, liking to dominate, but he is not nearly as violent and aggresive in exercising that personality trait.

So I think its mainly an age thing. I know my friend was absolutely tearing her hair out and it really upset her. Its hard on everyone- the parent of the aggressive child and the other kids and parents who have to put up with it!