In Psychology class, we covered this under two theories advanced by Albert Bandura, The social learning theory (Bandura, 1977) and Bobo Doll study theory (Bandura, 1961). Human behaviour is shaped by what we observe, watch or read... basically we learn behaviour from the agents of socialization; family, peers, older guys, media etc. Children (humans) pay close attention to those they consider models and encode their behaviour then later they can immitate or copy the behaviour they have observed.
E.g Aggressive behaviour can be influenced by watching aggressive content and vice versa. Now this mixed with drug abuse and genetic exposure to psychopath behaviour; the result is obvious.