This page lists fan theories about the television series Hey Arnold!.
Phil & Gertie are Arnold's Parents
The Gist of It:Phil and Gerti are actually Arnold's parents, and his football head is the result of a birth defect made more likely by their advanced age when they had him. To cope with being bullied for his deformity, he constantly imagines himself living in a world where everyone looks odd.
Arnold believes that he is an orphan. In reality, he is actually the child of Gertie and Phil, whom he calls his grandparents. Gertie and Phil are mentally unstable and told Arnold that his real parents died in a plane crash so that he would not figure out that their old age is what gave him his health conditions. One is hydrocephalus, the reason for his oddly shaped head. He also suffers from Arnold Chiari Syndrome, hence why his name is Arnold; as he is bullied, he has imagined an imaginary world with imaginary friends. This is the reason they all have weirdly shaped heads. It makes him feel like he is normal, and no one can pick on him because of his head.
Deadpanned: This theory was basically disproven when Arnold was reunited with his parents in the new 2017 Hey Arnold!: The Jungle movie.