Shelbyville Manhattan and Jebediah's split occurred when Jebediah refused to found a town where cousin marriage was legal. Shelbyville considered his cousins "so attractive" he thought the whole point of their journey to found a new town was for that reason. They went their separate ways, and founded their own cities. The cities have been rivals ever since.
- Though Jebediah Springfield did not support inbreeding, the Spuckler family live in Springfield instead of Shelbyville.
- Milhouse's parents are from Shelbyville originally, explaining why they look the same.