Hey Arnold! Running for nearly eight years, one key point was never clarified: we never actually met Arnold's family. That makes this crazy Hey Arnold! The theory about Arnold's parents, in which we are led to believe that Arnold's grandparents were actually his biological parents and that they lied to him, is even more disturbing. The theory cites mental illness as a reason why Phil and Gertie lied to their biological son, and even offers a medical explanation for why Arnold looks the way he does. Basically, it's going to shake any fan, no matter how casual, to their core and raise more questions about the founding of the Shortman family.
According to show creator Craig Bartlett, "The story of Arnold's missing parents unfolds like this: When I first pitched 'Hey Arnold!' to Nickelodeon, I was so excited about their "It was very glib." Initially, he claimed they were "going to Africa or something" to make a wildlife documentary, which is why the main character had grandparents as roommates. But when kids started writing letters asking what happened to Arnold's parents, Bartlett wrote the Season 5 episode "Parents' Day." In it, we learn that his parents left for a humanitarian mission in the jungle and were never heard from again. (It is presumed that their plane crashed, although Bartlett intentionally left it open as their plane was never actually found.)
He had further plans for the storyline and even came up with a movie plot in which Arnold attempted to travel to Central America to find his parents . Sadly, the film was never made (then being the operative word here). However, Hey Arnold! Fan theories suggest we've been hoodwinked all along. Phil and Gertie were not grandparents, but parents who gave birth to offspring at an advanced age. This descendant is Arnold.
According to Creepy Pasta (quoted from teen.com, as the original Creepy Pasta page regarding this theory has been deleted):
"The grandparents were mentally unstable and told Arnold that his real parents, of normal age, died in a plane crash. This is simply not true. Because Gertie and Phil were already very old when they 'created' Arnold, he was born with One of those conditions is hydrocephalus, and that's why Arnold had a form of hydrocephalus, which is why Arnold Chiari Syndrome."
If you've seen "Parents' Day," then you know that this is actually somewhat of a theory. The episode begins with Arnold being drawn into an elaborate bedtime story about his parents—about their adventures in the jungle and their time as acrobats in the circus—which makes the so-called “real” story Got to be even more suspicious. Like other anecdotes Phil tells, these are common but don't entirely make sense.
According to "Grandpa", Arnold's father was indeed a scientist, and he did meet Arnold's mother in the jungle. But when the couple returned to the United States, he was visited by his father's friend Eduardo, who said he needed their help because his tribe was dying from a mysterious disease. Then they took a plane into the jungle in the rain and were never heard from again. But at the beginning of the "real" story, when we see what appears to be a flashback, it's implied that Eduardo thinks Arnold's father is always in the clouds and that he's romantic and unrealistic. This is further reinforced by the alleged father's constant comments about the sky and falling off a rock face, thus requiring medical attention. It is difficult to imagine Eduardo sailing away to an entirely different country to seek salvation from a man he believed to be utterly incompetent.
Then, when Arnold's grandparents have spent the night dreaming of all the ways they could support him, they feel a strange sense of joy when Arnold suggests they return to the school-sponsored parent tournament. "Really? But I thought you said it was for real parents?" Phil asked. Arnold replied, "To me, that means you and Grandma." Gertie was so ecstatic that she climbed onto the kitchen table.
There's also the hydrocephalus angle, which provides an explanation for why Arnold alone (suggesting his unique features rather than a cartoonist-esque quirk) has such a wide head. According to webmd.com, the main symptom of hydrocephalus is that the head is "larger than normal."
Still, the medical aspects of the theory also weaken it. According to Australian charity The Brain Foundation, if left untreated hydrocephalus can lead to disability or death; health24.com supports the idea that hydrocephalus needs treatment and highlights the importance of medical support if you have the problem. It seems unlikely that Arnold had the disease and was not receiving treatment, which is no big deal. After all, he looks perfectly healthy.
So, no, this isn't the most convincing explanation in the world for where Arnold's parents are. But for a thought, you might want to grab a few beers with your same Hey Arnold! Obsessing with your best friend, if only to break their heart? Unparalleled.
