People have always considered boys to be naturally aggressive, but even this point is controversial. Dr. Charles Flatter is Director of the Department of Human Development at the Institute for Child Study at the University of Maryland in College Park.
Little girls can be aggressive, too, but the aggressiveness takes different forms.
Think of Lucy in the 'Peanuts' comic. Every kid is going to be different. The important thing is to temper aggressiveness with empathy. The world in which little boys are growing up today is more complex than the world of Wally Cleaver. Allow them to be boys while channeling their energy and instincts in nonviolent ways.