When children are infants, they do not think as much as they react to stimuli. For this reason they have a relatively high reactive aggression which is the aggression that happens as a result of person responding to a situation that they habor negative feelings about. This is why infants cry so much.
As they grow older however and pass the 2 year mark, they begin to think more about the situations they are in rather than react to it. They can be reasoned with and so they display less reactive aggression.
