Dib isn't human

Clone
Due to Dib having having a similar appearance to his father, some believe that Dib is a failed cloned. The side effects being his large head and lack of interest in real science.

Alien
Because of Professor Membrane constantly turning down Dib's obsession with the paranormal, some believe that Dib is in fact a paranormal being. The fear of Dib finding out, and possibly trying to find his alien parents has driven Professor Membrane to constantly try to steer his son away from the paranormal.

This also opens up the possibility that Professor Membrane is an alien, because of how advanced his technology is.