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* In the ''Disney/{{Aladdin}}'' sequels and [[WesternAnimation/AladdinTheSeries series]], Genie seems to have lost brain cells as well as magic powers. He frequently [[ForgotAboutThePhlebotinum forgets about the phlebotinum]] and has IveHeardOfThatWhatIsIt moments about things he should have already encountered (e.g. he asks what a royal vizier is, when a royal vizier was the BigBad in the original movie!) The fact that he [[RunningGag keeps losing games against a rug]] doesn't help either.
* ''WesternAnimation/AmericanDad'': Ever since Season 9, Stan Smith slowly seems to lose more and more brain cells to the point where a more subtle name for him as this point is Peter Griffin II.
* ''WesternAnimation/TheAngryBeavers'': Daggett was never exactly clever, but as it went on his intelligence decreased sharply. He stayed angry, though.
* The ''WesternAnimation/{{Battletoads}}'' cartoon turned the three heroes into brain-dead teenage idiots. It even introduces Morgan "Zitz" Ziegler, a GeniusBruiser in the game canon, by having him cause a computer to explode just by trying to type something into it.
* ''WesternAnimation/BeavisAndButtHead'' started out as just stereotypically lazy and not all that bright teenagers. As the show's popularity picked up the need for more outrageous material resulted in them becoming the TooDumbToLive types everyone knows them as. Beavis especially went from a generic dumb teenager to a bizarrely stupid boy with a split personality.
** Interestingly enough, it appears to be inverted during the duo's music video commentaries where they actually seem brighter than normal.
* ''Franchise/Ben10'': while Ben was immature and bratty in the original show, this was justified due to him being a 10-year old kid. When the sequel ''WesternAnimation/Ben10AlienForce'' turned him into a teenager, he TookALevelInBadass and became much more competent, smarter and responsible. Then in season 3 the writers tried PanderingToTheFanbase in a failed attempt to make him more like his 10 years old self, and he started acting more narcissistic and cocky again. Partially justified in-universe in that saving the Universe went to his head, but it sticks far longer than it should. This was then zig-zagged in ''WesternAnimation/Ben10UltimateAlien''. Finally, ''WesternAnimation/Ben10Omniverse'' took the trope to {{Flanderization}} levels on both his teenage self ''and'' his 11-year old self, making them both much more bumbling, clumsy and immature while removing a large amount of his badassness from ''Alien Force''.
* ''WesternAnimation/TheBrakShow'': Thundercleese started off as the badass neighbor. As his appearances became more common, his intelligence lowered but not so much that he had no common sense. This changed in [[SeasonalRot Season 3]] when he became so stupid that it's a wonder he was still functioning. Want proof? He fell in love with ''and married'' Brak's Dad's vacuum cleaner.
* Onua from ''Toys/{{Bionicle}}'' was originally the ''wisest'' of the Toa, but you wouldn't know that from watching the DirectToDVD film ''Mask of Light'', which chose to portray him as a slow-minded muscle-man who liked to chat about "real jobs" like mining. The dumbass part truly comes in when one of his attacks causes a cave-in that destroys his own village and almost kills him along with his entire team. This directly contradicts his bio, which states that he always considers every option in battle, and chooses the one that's least dangerous for his friends, and is also at odds with his actions in the comics, where he was always the one ''doing'' the saving. Thankfully, he went back to being a GeniusBruiser after the film.
* ''WesternAnimation/{{Chowder}}'': Chowder; compare the naive little boy of season 1 to the completely TooDumbToLive of season 2.
* Cleveland grew progressively dumber when he left ''Family Guy'' and starred in his own show, ''WesternAnimation/TheClevelandShow''. Cleveland used to be the voice of reason in Peter's group but once he moved away. Now he is pretty much a black version of Peter.
* Wallabee Beetles aka Numbuh Four of ''WesternAnimation/CodeNameKidsNextDoor'' in the first couple seasons his stupidity was an InformedFlaw, the villains often referred to him as "the stupid one" when in actually he wasn't all that dumb, just a little less intelligent than Numbuhs One and Five, at times he would let his anger get the better of him but he was very resourceful and had some brilliant tactics to his plans. In later seasons he's so stupid he can't spell or do basic math, he's a lot less competent and badass than he originally was, he very seldom won fights anymore, and often whined when things didn't go his way.
* Penfold on ''WesternAnimation/DangerMouse'' was the token dumbass, but as the series wound down to its final two seasons, Colonel K was afflicted by a case of the dumbasses which could be chalked up to early senility.
* ''WesternAnimation/TheFairlyOddParents'': Pretty much every character with the exception of Wanda and A.J. Timmy started off simply as a ten year old boy who usually made rash decisions without thinking his wishes through. The newer series makes it debatable how he hasn't destroyed the world already yet.
** Cosmo most definitely. His first appearance was of a suave, gentlemanly persona in the first episodes, who made sarcastic comments and wisecracks. Now he's a screeching, literally brainless fairy who would have already killed Timmy if not for his wife's constant surveillance.
* ''WesternAnimation/FamilyGuy'': Peter Griffin was a dumbass to begin with yet he was a good guy at heart and after discovering how much trouble his idiocy caused would do his best to fix it. After the [[UnCanceled uncancellation]]. Now he's gotten even worse. He's legally retarded in-canon. You never would have seen that coming in the show's early seasons. Taken UpToEleven in "Brian's a Bad Father". Quagmire calls Peter out for foolishly shooting him in the arm during a hunting trip. Heck, now he can't even walk and chew gum ''at the same time''.
* In season one of ''WesternAnimation/FriskyDingo'', the villain Killface is intelligent, educated and sophisticated, by contrast to the IdiotHero Awesome X. A lot of the comedy from Killface stems from the contrast between their personae. Come season two, they're both complete morons.
* This happened to Fry in ''WesternAnimation/{{Futurama}}''. In the earliest episodes, where he was supposed to be a FishOutOfTemporalWater and stories would mostly focus around him adapting to life in the 31st century, he was a fairly average guy with a dash of {{Cloudcuckoolander}} and a few moments of genuine cleverness. As the show progressed, he adapted to his surroundings much faster than the writers intended, and so he devolved into a gibbering manchild [[PingPongNaivete unless the writers needed him to say something profound]].
** It's somewhat justified in-universe with Fry's lack of a delta brain-wave, which effectively stunted his mental growth:
--->'''Professor Farnsworth:''' And, Fry, you've got that brain thing.
--->'''Fry:''' I already did!
*** Of course, the means in which he contracted it as shown in the episode "Roswell That Ends Well" would have meant [[spoiler:[[GrandfatherParadox he had this his whole life, including all prior episodes.]]]] Also of note is that, in "The Why of Fry", [[spoiler:the Niblonians reveal that they deliberately froze him so that his "superior, yet inferior" brain would be used to protect the universe in the 31st century. Later it is revealed that the Niblonian who blew Fry into the cryochamber was, in fact, not a time traveler, and that the Niblonians knew of this phenomenon at least a millennium before the Roswell time travel incident.]]
*** [[spoiler: It's possible the Nibblonians only knew about him a millenium in advance after he did the whole Roswell timeloop. Time travel is fun like that.]]
* The titular WesternAnimation/JimmyTwoShoes in season 1 was an airheaded and oblivious fellow, but he was shown to have genuine moments of cunning [[TheCloudCuckooLanderWasRight (if odd ones sometimes)]]. [[DependingOnTheWriter Some writers]] went even as far as making him the StraightMan to [[FatIdiot Beezy]] and [[MadScientist Heloise]]. In season 2, he's as dumb as Beezy, even more childish and oblivious, and more prone to making things worse due to his own idiocy [[UpToEleven than ever]] with not a single moment of intelligence in sight.
* ''WesternAnimation/JohnnyBravo'': In the first season, Johnny was just naive and unable to understand why his CasanovaWannabe approach doesn't attract women. He was shown cooking barbecue and had managed to outsmart a leprechaun and a Film/JamesBond style villain. His intelligence took a nosedive in seasons 2 and 3, where he couldn't even prepare cereal without his mother's help. Averted when the series was temporarily revived, with Johnny instead trying to avoid his now-annoying neighbor, Suzie.
* ''WesternAnimation/KingOfTheHill''
** All of Hank Hill's friends seem to slowly lose their precious remaining IQ points over the course of the series -- resulting in dumber and more outrageous schemes that Hank must eventually fix.
** Luanne and Peggy also. Peggy has never been the CloserToEarth person between her and Hank, but she used to be at least a competent Spanish teacher -- now she's shown to have a tenuous grasp at best on the Spanish language, though it does lead to some [[CringeComedy admittedly hilarious moments]]. Luanne has had a similar path of regression. She was a bit of a WrenchWench during the first two seasons, but that was eventually abandoned and she became increasingly ditzy as the years went on (it should be noted that [[{{Misblamed}} this happened long before Lucky came along)]].
*** Peggy was bad at Spanish at the beginning of the series, in fact, she actually got a little better as it went on. The rest here is spot on, though.
*** Actually, Luanne could be seen fixing cars in later seasons, too.
** Joseph was a little kooky, being Dale's son and all (by nurture, of course) but when he hit puberty, he pretty much only begun to think with his hormones. This to the point where his dialog was replaced with HulkSpeak.
*** It got to extremes in one episode where Bobby accidentally sees Luanne without a shirt on and Joseph practically went beyond creepy levels when he kept asking Bobby to look at Luanne topless again so he can get the details on what her breasts look like.
** Inverted with Bobby, refreshingly, who went from slow, dull, and lethargic to... Well, not exactly smart, but more definitely charismatic and possessing of a decent amount of common sense.
* Literature/PaddingtonBear in the 1990s TV show by Cinar. Sure, Paddington [[TheDitz wasn't always the sharpest knife in the drawer]], but the number of WhatAnIdiot moments he exhibits in this particular series (which includes him taking "tying off (the boat)" to mean tying ''himself'' off by wrapping a rope around himself (thus causing a boat to float away) and making [[https://en.wikipedia.org/wiki/Beef_Wellington Beef Wellington]] ''with his own Wellington boots'') is rather glaring.
* ''WesternAnimation/ThePowerpuffGirls'':
** Fuzzy Lumpkins took this between the pilot and the series proper: in the pilot he was a reasonably calm [[BuffySpeak furry... thing]] with enough intelligence to build a gun that turns objects into meat; in the series he's a trigger happy hillbilly stereotype whose idea of a weapon is a rock (compared to, say, the ray guns of Mojo Jojo and Princess). Consider his line in "Fuzzy Logic" while chasing a squirrel from his home:
--->'''Fuzzy:''' Come back here an' git off mah proppity!!
** The Mayor was never the sharpest knife in the drawer but in the early seasons he seemed to be more naive rather than outright stupid. By the end of the series half the time he can't even structure proper sentences together. Highlight to an episode where Professor Utonium has to literally ''babysit the Mayor'' when he drops by.
* Peter Venkmann from ''WesternAnimation/TheRealGhostbusters'' when his original voice actor Lorenzo Music was replaced by David Coulier his wit and intelligence steadily decreased to the point where he became a bumbling, catchphrase spouting, buffoon who was now Slimer's best friend, whereas in the earlier seasons he was often annoyed by his antics and wished he would leave him alone.
* T.J. from ''WesternAnimation/{{Recess}}'' started out relatively average in the first season. He became more BookDumb in season two, and by season four, he was TheDitz. Though to an extent, [[TropesAreNotBad this was for the better]], as he was a borderline MartyStu in the early episodes.
* In the 1965 version of ''WesternAnimation/SecretSquirrel'', Morocco Mole was a BumblingSidekick. In the 90's revival, ''Super Secret Secret Squirrel'', he's TooDumbToLive.
* ''WesternAnimation/TheSimpsons'':
** Homer Simpson has never been the brightest bulb in the box, but he actually started out a fairly normal person. The trend towards increasing dumbassery was so noticeable that more than once there have been in-show explanations for this, all contradictory. Creator/ConanOBrien said on a DVD commentary that Homer's IQ started out at 65 and dropped 5 points a year until it hit zero in Season 13.
-->'''Frank Grimes:''' I don't know anything, because I'm Homer Simpson!
** Ralph Wiggum has suffered from this trope more than any almost any other character mentioned. He was originally just another generic classmate of Lisa's before becoming the unbelievably dense collection of non-sequiturs we all know. WordOfGod says that Ralph was originally intended to be an eight year old version of Homer. When they realized that didn't really work, they just made him the most childlike character imaginable.
** Much like Homer before him Bart has become increasingly stupid in recent seasons, in earlier seasons he didn't do well in school mainly because he didn't care and he couldn't stay focused for long periods of time but he was brilliant in other fields as he could manage to pull off time consuming schemes and pranks in minutes, could master languages in relatively short time, and was a skilled detective, in recent seasons he's so stupid he can barely read or write, is easily entertained, and has trouble figuring out simple tasks.
** Marge has also become less intelligent with every season. Initially introduced as the straight man to Homer's antics (yet NotSoAboveItAll at times), later episodes had her eagerly play along with Homer's idiocy and say stupid things just as much as her son and husband. Doesn't help that she also TookALevelInJerkass too with her niceness and good parenting becoming an InformedAttribute. As one will have observed, the entire purpose of this family IQ degradation is too make Lisa the OnlySaneMan[=/=]force viewers to feel bad for her.
* ''WesternAnimation/SouthPark'': Almost every single adult undergoes this to a degree, but Randy Marsh is possibly the greatest example of this trope ever. Through all seasons of the show, he has taken more levels in dumbass than any other character, other than possibly Ralph Wiggum. Despite being the town's only scientist, no less.
* ''WesternAnimation/SpaceGhostCoastToCoast'': Done with several characters.
** Brak's case was actually justified; the cloud of space dust into which WesternAnimation/SpaceGhost threw him during an episode of the original series was apparently to blame.
* WesternAnimation/SpongeBobSquarePants has always been [[{{Cloudcuckoolander}} a little loopy]]. But he has become actually dumber and dumber (not just loopier and loopier) as the series has progressed. Especially after "A Pal for Gary". You'll hate him by the end.
** This trope also applies to [=SpongeBob=]'s best friend, Patrick. Oh so much... Though, truthfully, how much dumber can you get after mistaking mayonnaise for a musical instrument? Or forgetting that you ate a candy bar literally ''two seconds later''.
* ''WesternAnimation/TeenageMutantNinjaTurtles2012'': Michelangelo is easily the least intelligent of the Ninja Turtles due to his childlike personality, but in spite of this still managed to showcase some hidden smarts, but as the show went on you have to wonder how he can function sometimes. Compare a first season episode where he's able to successfully able to read Donatello's notes to make a cure for a mutated venom that's turned his other brothers into mindless zombies, while being infected with the same venom no less, to an episode one season later where he willingly exposes himself to potentially fatal mutagen while even ignoring the warnings on it, nearly dying as a result.