Superman is too good a role model. Fans of the man from Krypton unwittingly compare themselves to the superhero, and realise they do not measure up. And as a result, they are less likely to help other people.
The one paragraph that struck me is the following:
Students who thought of Superman volunteered much less of their time than those who thought about other superheroes. Furthermore, Superman-primed subjects were significantly less likely to show up at a meeting for volunteers held three months after they were initially asked to participate.
Are you thinking about superheroes day to day? Hmm.
The reason, believes Nelson, is that asking people to compare themselves to an exceptional individual makes them realise their shortcomings. Whereas thinking about a general category encourages people to identify the strengths they have in common.