There is a proven theory in psychology that a (typically repeated) fantasy about performing an action has a similar effect on the subconscious as actually performing the action itself; this works in good ways as well as bad ways because a psychologist can use it to help someone with their phobia through focused imagination, at the same time though child pornography allows paedophiles to build their appetite and cross boundaries which would be less likely in the absence of child pornography.
Now, I'm not sure that the ESRB is focusing on the right thing with their ratings ... I do not believe that the graphic nature of the outcome is nearly as important as the action that is performed itself.







