Are bullies born or raised? The age-old nature versus nurture debate is alive and well in those who study aggressive behavior. On one side of the debate, nature researchers believe that heredity and hormone levels in the human body can cause increases in aggressive behavior. The nurture crew believes in the blank-slate theory — a child's understanding of and reaction to the world come entirely from his experiences and observations. Many researchers now believe that while biology can cause certain predispositions, environment plays a significant role in the development of aggressive behavior.