Here's a theory I have about the relationship between the Human and Monster worlds:
At some point, the "monsters", whose technology seems to be a bit ahead of ours, discovered how to create the portals into our universe. But humans feared them and tried to destroy them, so contact between the worlds was forbidden. Eventually, humans just forgot about their existence, except as fairy tales to scare children into behaving.
Meanwhile, the monsters ran out of fuel sources like oil. They then rediscovered Earth, and that human children produced the energy they needed from their screams. To keep unscrupulous people from just kidnapping children, the good monsters in their government made up the "humans are toxic!" lie, and Rozz's agency was meant to keep it going (and still does!)
Also, in the process, some monsters have been importing the fashions of human culture (everything from clothes to cars) into Monstropolis, without telling its citizens where they were invented. Maybe our cultural diversity is our one advantage over them.
edited 25th Jun '12 6:38:44 PM by Sijo