Phlebotinum-Induced Stupidity

A device or effect that makes people less intelligent. (Usually temporarily.)

Generally played for comedy, with the afflicted people talking in exaggerated "stupid" dialect and making decisions that are a Take That against whatever the writers oppose. In almost all cases, the effect hits both "book smarts" and "common sense", even though they're different kinds of intelligence. For extra fun, one character will be functionally immune to the effect due to already being stupid.

Can be used to justify the Idiot Ball, as opposed to Plot Induced Stupidity, which does not bother with an explanation.

When someone's using this as a weapon, it's a type of Stupidity Inducing Attack.

Examples:

Comic Books

 * The Judge Dredd comic had an experimental weapon named "the Stupid Gun." It fell into the wrong hands--Hilarity Ensues.
 * In Super Mario Bros #5 (from Valiant Comics), King Koopa's (full name "Bowser Koopa") science team comes up with "the Stoopid Bomb." The King of the Mushroom Kingdom, being an idiot, is unaffected, but everyone else becomes foolish, including King Koopa's own troops, as the bombs are on a hair trigger and keep getting dropped. There are also antidote Smart Bombs, but they are all used on one Shyguy, who becomes a genius and temporarily wrests control from Koopa. Too bad he's now the Only Sane Man.
 * The end of the President Evil arc in the DCU explained Lex Luthor's blaming Superman for a kryptonite meteor heading for Earth, putting on Powered Armor, and ranting madly on national TV, by him having injected himself with a mind-altering mixture of Bane's Venom formula and liquid kryptonite. Why he decided he needed some pretty low level Super Strength despite being the President of the United States is definitely Plot Induced Stupidity though. Damn you Loeb.
 * Drinking green, radiated rocks from outer space in liquid form is even worse. And don't forget this IS the guy who had cancer because he wore a Kryptonite ring all the time.
 * Geoff Johns later threw in a revelation that Lex's recent bouts of stupidity were caused by the presence of his Alternate Universe 'counterpart', Alex Luthor Jr. Somehow. Even though he is actually the son of his universe's Lex.
 * In the utterly depressing alternate Marvel Universe that serves as the setting for Mutant X, there is a Dumb Muscle character called The Brute. He used to be brilliant scientist Hank McCoy before his notorious Professor Guinea Pig tendencies took a tragic turn, leaving him too stupid to figure out how to reverse what he'd done to himself.
 * Maul of the comic book Wild C.A.T.S has size-changing powers that makes his intelligence inversely proportional. Not only does growing make him dumber, he later discovers he can shrink to get smarter, although this proves exhausting.

Literature

 * There was at least one Airport Fantasy-style disaster novel which had this effect as the result of a communicable disease. Toward the end, a newscaster (who is having trouble reading the TelePrompTer) notes that sales of all literature except pornography have fallen drastically. The heroes finally found a cure, but were uncertain if anyone would want it anymore.
 * This sounds like IQ 83 by Arthur Herzog.
 * "Bimbo Rays" are popular in certain kinds of fetish fiction.
 * You heard this right. It's often combined with Professor Guinea Pig and Gender Bender in most circles.
 * "AUM" in the Illuminatus! trilogy is a hallucinogenic drug cocktail that renders users highly creative and supremely gullible at once. When one character tests it by spiking the punch at a meeting of a Fictional Counterpart to the Knights of Columbus, the group ends up renouncing Christ and adopting geocentrism.
 * Unlike most examples, this stuff tends to make people more intelligent, not less. Initially they believe almost everything that follows some kind of logic, but since it stimulates inquisitivness and curiosity, it tends to produce more knowledgeable and open-minded people, on average. It's implied that sometimes the opposite can happen as well, but those examples are never seen in the book.
 * In the short, short story I Wish I May, I Wish I Might by Bill Pronzini, a retarded boy finds a genie in a bottle which offers to grant him three wishes. One of them? "I'm going to wish for all the little boys and girls in the world to be just like me so I'll never-ever be without somebody to play with."
 * An example where the change is permanent, the Stephen King short story The End of the Whole Mess, was about some Phlebotinum applied to the world's water supply in order to make everyone less aggressive. It worked, but it caused the end of the world anyway, because a side-effect was rapid onset dementia/Alzheimer's.
 * The novel Bimbos of the Death Sun is centered around the author of a novel within a novel with the same name, whose basic premise involves solar radiation messing up computers and (as a side note) making women lose intelligence. Despite what it sounds like, he wasn't doing it to be chauvinistic - he's an electrical engineering professor and the phenomena were based in his research.
 * In Robert Forward's Rocheworld, the ship's crew are given a drug (NoDie) that greatly extends their lifespans at the expense of making them stupid. The ship's AI stops administering the drug when they get to their destination, and their intelligence comes back.

Live Action TV
"Xander: And was there a lesson in all this? huh? What did we learn about beer?
 * The Buffy the Vampire Slayer episode "Beer Bad." Buffy drinks cursed beer which turns her into a cavewoman. (Although she still looks the same, the guys who drink the beer literally look like cavemen. Granted, Xander expressly cut her off before the rest) Ends with a Spoof Aesop:

Cave Buffy: Foamy.

Xander: Good, just as long as that's clear."

"Giles: I can't believe you served Buffy that beer.
 * Of course, the beer itself wasn't a good idea for someone who can literally punch through car doors.

Xander: I didn't know it was evil.

Giles: You knew it was beer!"


 * Another Buffy episode had Ethan Rayne selling chocolate that made people act like particularly irresponsible teenagers. This is notable because "book smarts" weren't affected so much as common sense was. Buffy wound up having to be the responsible, mature one.
 * It seemed to depend on what sort of person the reverted individual was as a teen. The principal was, and became again, a dweeb and mindless hanger-on to whoever seemed coolest (in this case, Buffy.) Giles was, and became again, a danger to everyone around him.
 * In the Eureka episode "E=MC...?", eating cloned chicken meat causes most of the town to lose their knowledge, common sense, attention span, emotional maturity, etc.
 * Monty Python's Flying Circus - In the "Gumby Surgery" sketch, the head surgeon (Graham Chapman) is perfectly intelligent and lucid while asking the nurse for glasses, mustache, and handkerchief. As soon as the latter is put on, he turns into the usual Gumby idiot/madman: "I'm going to operate!"

Toys

 * One of the Vahki models in Bionicle had the ability to temporarily overwrite a Matoran's conciousness and turn them into a mindless "shambler" incapable of rational thought.
 * It also occured to Vezok when he got a Literal Split Personality - the original Vezok lost a few IQ points because the part split off got his ability to think tactically.

Video Games

 * In the game Psychonauts using super-powerful sneezing powder to make people literally sneeze their brains out results in the person turning into a listless zombie obsessed with television (and hacky-sack, for some reason).
 * The trope is also inverted in the case of Ford Cruller, who (due to a crippling psychic battle) is schizophrenic and amnesic when he's not near the Psitanium deposit.
 * The plot of the point-and-click adventure game Zak McKracken and The Alien Mindbenders revolves around aliens who took over the phone company and are sending out a signal that makes people stupid.
 * In Soul Nomad and The World Eaters,

Webcomics

 * In the first Djinn arc of The Wotch, a wish turns all girls except blondes into stupid bimbos.

Western Animation

 * In Futurama, Fry is the only one immune to the giant brains' stupefying effect due to his...unique...brain structure, which is normally a handicap.
 * I already did!
 * No I'm... doesn't.
 * In an episode of Chalk Zone, Craniac 4 tries to get Rudy with a smart bomb. It is sentient, though, and does not want to blow itself up. He instead opts for the Dumb Dart, which lowers intelligence.
 * An episode of Kim Possible had Dr. Drakken using "silly hats" to reduce the world's leading scientists to blithering idiots.
 * In one episode of Sonic the Hedgehog, Dr. Robotnik accidentally takes a Stupid Pill instead of a Smart Pill.
 * There's an episode of Jimmy Neutron where Jimmy builds a "brain drain" helmet to lower his intelligence to normal levels. Unfortunately, the helmet works a little too well and he becomes a blithering idiot.
 * Spongebob SquarePants: "You won't get away Man-Ray; not while we have the Orb of Confusion!" Which, by the way, causes a wave of confusion that starts with the person who turned it on, making it a very literal Idiot Ball.
 * An episode of Rocky and Bullwinkle featured "goof gas". Bullwinkle was too stupid to be affected by it. Boris remarked something like, "Goof gas affects the brain. No brain, no effect."
 * Pretty much the exact same thing, with different Phlebotinum, happened in The Movie.
 * An episode of Invader Zim had a power amplifier that "radiated pure stupid" when GIR hooked himself up to it.
 * In the Re Boot episode "Enzo the Smart", Enzo messes with the clock speed of Mainframe in an attempt to make himself smarter. He asks it to make him twice as smart as everyone. Instead of becoming smarter, everyone else becomes half as intelligent as him.
 * Homer Simpson's stupidity isn't natural: a crayon lodged in his brain takes 50 points away from his previously average IQ.
 * This is worsened by the fact that the nuclear power plant where he works doesn't use proper shielding. All the slapstick violence in the form of blows to the head certainly can't help, either. Word of God is that he loses 5 IQ points a season from brain damage, and at this point he's basically "a dog that can talk."
 * Used as a brief gag at the end of one episode of Buzz Lightyear of Star Command. Robot Buddy XR has had files he's not supposed to have on his brain chip throughout the episode. At the end he deletes them. However, he also erases most of his IQ by mistake.
 * Played with in an I.M Weasel Short on Cow and Chicken, in which I.R. Baboon managed to publish a book that apparently turns everyone who reads it into complete and utter idiots. Lamenting on this turn of events, I.M. Weasel decides to expose himself to the "stupid powers"... but remains unaffected. He eventually realizes that everyone around him was already a stupid twit and he just didn't notice it.
 * Lilo and Stitch The Series has Experiment 319, named Spike, who can spray spikes that raises peoples' sillines by 99%, leaving them only 1% clever.
 * In The Penguins of Madagascar episode "Sting Operation," the penguins use one of Kowalski's inventions to make themselves stupid
 * In another episode, Kowalski's attempt to boost his intelligence backfires and makes him stupid instead.
 * Susan and Mary Test built an intelligence amplification machine, and Johnny used it to give them an IQ of 22.

Real Life

 * Ethyl alcohol.