When my son, "Jack," started preschool as a 2 yr old, he was a very pleasant child, with no aggressive or hyperactive tendencies whatsoever. In the 2 yr old class, he met a boy, "John," who is very aggressive. A typical act was for John to pull Jack's hair (no provocation) til he almost pulled him to the floor, then run away laughing. I saw this for myself. Jack started hitting, & then went beyond John to include other kids.