New Watson: JAM!
Old Watson: Holmes, please....
Holmes: New Watson likes jam. We're very happy.
A specific subtrope of Flanderization (and occasionally Jerkass also). As a series progresses, characters sometimes become more and more stupid as time goes on. Usually the reason is related to the Badass Decay phenomenon, but with the character's level of intelligence rather than his ability to kick ass, following something along the following cycle: an episode in which the characters acts like a total buffoon gets good ratings, so another one is made. It too gets good ratings. So they make another. And another. Eventually the character is so dumb that you wonder how he manages to walk across a room without injuring himself, and pretty soon everyone forgets that the character was ever anything other than a complete moron. For this very reason and the fact that it's easier to write comedy that involves dumb people, this trope is rather frequent in any comedy series.
Generally Played for Laughs. Sometimes used on the supporting characters to make the hero look more competent. Or sometimes, it's just that the show gets a pack of new writers and they turn the guy into a moron for kicks. It should be noted that this isn't always a bad thing. In some cases, viewers may find the over the top idiocy more entertaining than the original concept, or that giving them some idiotic traits make them more of a Rounded Character.
May result in a typical Idiot Hero degenerating into Too Dumb to Live. Compare Forgot About His Powers and Took a Level in Jerkass. Contrast Took a Level in Badass and Dumbass No More. If this happens when one works gets adapted to another medium rather than occurring within the same work, see Adaptational Dumbass.
- Happy Heroes: In Season 8 episode 4, Big M. uses a special powder that makes any affected targets dumber. He uses it on Sweet S. so that he can get the fire wand from her, but then he needs help from drowning in a big cauldron and all nearby sources of help, including Little M. and Huo Haha, completely fail to actually get him out of the cauldron since they drank water contaminated with the powder. Oops.
- Dilbert: The defining character trait of Dilbert's boss was a result of this. In the earlier strips he was just a regular Bad Boss who was more likely to be shown performing evil experiments on his employees than losing at chess to a pineapple. And done deliberately in one strip where a new guy transitions from appearing to be smarter than everyone else in the team to having neanderthal-level intelligence over the course of a three-panel comic.
- FoxTrot: Roger, the father figure. In the earliest years of the strip, he was a competent enough patriarch, though a bit harried and somewhat of a Small Name, Big Ego. As the strip went through Reverse Cerebus Syndrome, his intelligence dropped severely and his occasional bouts of incompetence (he was a Walking Techbane from the first month onward) became exaggerated to match.
- Garfield: Jon Arbuckle, Garfield's owner. He started as a perfectly ordinary young man. Come the early-2000's, and he's a total moron who finds interest in mundane activities, mistakes a sudoku for a crossword, and mistakes an upside down restaurant menu for French. It was around the same time he was Flanderized into being the Straw Loser to Garfield.
- Peanuts: Sally Brown started out as reasonably naïve and childish before devolving into a full-on The Ditz who made hilariously inaccurate school reports.
- Twilight and her friends in My Brave Pony: Starfleet Magic. When meeting Rhymey during his visit in Equestria, they all mistake him for a crossdressing Fluttershy. Bonus points go to Rarity, who outright states she has no problem that "she" completely transformed herself with magic into a male. The remake tries to address this by making them believe that she is disguised, which manages to make them look even dumber.
- Yang has a mild case of this in React Watch Believe Yikes. She's generally a bit slower on the uptake compared to the others and is less book-smart than in canon.
- 8-Bit Theater:
- Bikke the pirate was actually fairly ruthless and cunning when he first appeared (well, compared to Garland and the heroes, anyway). At this point, his intelligence is viewed as on par with Fighter's. In fact, he's probably worse since he's not even literate.
- D'rizzl actually inverted this trope, by taking levels in Smartass. Complete with a 50-point IQ hike.
- Nick in Schlock Mercenary, who went from "stereotypical dim-bulb grunt" to an extreme ditz.
- In Sluggy Freelance Sam "Da Man" Sein's intelligence has been plummeting ever since he became a vampire.
- Fighter himself could also be considered a victim of this trope. Initially, he was simply The Fool with an intense sword obsession. For the past few hundred comics, he's essentially been a total idiot.