In explaining an individual's aggressiveness, skinner would look for

I believe the answer is:  early learning experiences and reinforcement history.

According to skinner, the level of aggressiveness that people show is a direct result of the accumulation of their past experience when they tried to get something what they want.
People tend to show higher level of aggressiveness if the people around them constantly cater to their needs and fulfill their wishes without giving them any challenge.