Hey Arnold! introduced viewers to a boy with football-shaped head that lived with his grandparents in an inner-city boarding house, and these details made way for a very dark theory about the main character. Nickelodeon took over televisions all over the world in the 1990s with its kid friendly content and cartoons, collectively known as Nicktoons, and one of the most successful shows was Hey Arnold!.
Created by Craig Bartlett in the early 1990s, Hey Arnold! centered on the experiences of Arnold and his friends living in a big city and dealing with different types of problems that went from bullying to unrequited love, to more darker themes like the disappearance of Arnold’s parents – and this, along with the peculiar shape of his head, is the core of a very dark fan theory.
Like many other dark theories about cartoons, this Hey Arnold! theory originated as a Creepypasta, and even though it was deleted a while ago, nothing is really gone once it reaches the internet. The theory says that Arnold’s grandparents, Phil and Gertie, are his real parents, and because they had him at an old age, Arnold was born with a few health conditions, like hydrocephalus – hence the shape of his head. The theory went as far as to say that his name came from a form of hydrocephalus, known as Arnold Chiari Syndrome.
As a result, Arnold was bullied and had no friends, so all the characters in the series are a product of his imagination. The theory points out that the rest of the characters also have oddly shaped heads, and that Arnold imagined them as such so he could feel normal. The story supports itself by the fact that the location of Arnold’s parents was a mystery, and grandpa Phil always came up with different tales about them. This was finally solved in the TV movie Hey Arnold: The Jungle Movie, where Arnold's reunited with his parents.
The theory also added that Bartlett came up with the concept of Hey Arnold! after meeting the child of two elderly adults, who was mentally unstable and spoke about his imaginary friends and their adventures. Of course, this theory and every added detail to try to validate it are not true. Bartlett based the character on himself, as he was a daydreamer when he was a kid, and has publicly shared that he hates that theory (and the one about the Pigeon Man).
The theory originated after the series failed to explain what happened to Arnold’s parents, something Bartlett and company were ready to reveal when the series was suddenly cancelled, leaving a big plot hole for years. Hey Arnold! surely has some fun and weird characters with equally weird backstories, but they are all the product of the harmless imagination of the writers, and not of some twisted, dark experiences.