It's more important to what a person does because as they say, actions speak louder than words do. A person can think a certain way but not act on it but what a person does reflects what they decide internally and what they wanted to do with that idea externally showing that they are serious with that idea of theirs in their head.
Which of these would be an appropriate replacement heading for paragraph IV?
D) Detrimental Effects of MP3 Players on Society
Hope this helps!