The Hat Makes the Man

Sometimes, the type of hat a person wears can tell you a lot about their personality. However, these hats go a step beyond and noticeably alter the personality of their wearer.

The hat may be a form of personality-altering Applied Phlebotinum. Particularly bad cases may function more like Demonic Possession, with the hat completely overwriting the wearer's personality. Or the hat may simply be a Magic Feather, giving the character the boost of self-confidence necessary to alter their behavior. However it works, the character's behavior is markedly different when they wear it.

If the hat alters their behavior for the worse, then it's also an example of Clothes Make the Maniac. It has very little relationship to Planet of Hats.

If it's used by someone else to control the wearer it's also a Hypno Trinket.

Anime and Manga

 * One episode of Kirby: Right Back at Ya! was about an evil hat that can corrupt whoever puts it on.

Comicbooks

 * Doctor Fate's Helmet of Nabu would cause him to be possessed by the spirit of the deceased wizard, Nabu.

Literature

 * In Discworld, both the crown of a king and the Archmage's hat exert their own influence upon the wearer.
 * According to Cohen, so does the turban conventionally worn by the Grand Vizier. "Give 'em a turban with a point in the middle and it just erodes their moral wossname." This could just be a metaphor, though, as opposed to the Archchancellor's hat that literally has magical influence.

Live Action TV

 * There's a largely-forgotten Norman Wisdom sitcom from the 70s where his entire personality would change according to what hat he was wearing.
 * Linda's magic hat from the Round the Twist episode "Copy Cat", which grants the wearer the power to fly but also starts changing the wearer's personality.

Tabletop Games

 * Dungeons & Dragons
 * Helm of Opposite Alignment. Instantly changes the wearer's Character Alignment to its opposite (e.g. Lawful Good to Chaotic Evil).
 * The Crown of Souls will eventually change the wearer's alignment to Neutral Evil.
 * The Forgotten Realms Horn of Crowns causes its wearer to act in a crazed and violent manner.
 * Hat of Command: The wearer feels an overwhelming desire to take charge.
 * Hat of Occupation: Wearer thinks and acts as if he's a member of another class, but isn't.

Video Games

 * The Helm of Dark Magics in Arcanum permanently raises your magical aptitude and lowers your alignment each time you put it on.
 * The Peak Point helmets from Ape Escape. The most notable example is Specter, who goes from a cute and adorable monkey to a scheming, megolamaniac supergenius. The only exception to this is Pipotchi from 2, who wears a special pipo helmet designed by the Professor which increases his intelligence but keeps his lovable personality intact.

Webcomics

 * In one Skin Horse comic, an avian-humanoid freaks out because Tip put on a hat. She believes that hats make you evil and instantly worthy of hate.
 * Girl Genius:
 * When Agatha returns to Castle Heterodyne, the only way for humans to communicate with the castle's central brain is for the Seneschal to wear a helmet that inserts probes into his brain, allowing the Castle to take control of his body.
 * In the "Trelawney Thorpe: Spark of the Realm" short story, the MacGuffin is the crown of King Arthur, which is supposed to give the wearer the power of King Arthur.

Western Animation

 * The Looney Tunes cartoon "Bugs' Bonnets" has Bugs Bunny and Elmer Fudd switching personalities as various hats blow past and land on their heads.
 * The Care Bears Adventure in Wonderland, Grumpy Bear changed personalities every time he put on one of the Mad Hatter's hats.
 * In one episode of The New Adventures of Superman cartoon, a possessed hat causes anyone who dons it to commit evil acts.
 * Darkwing Duck several times had to fight alien, sentient mind-controlling hats.
 * Ruby Gloom's Skull Boy has problems with this thanks to a "Shakespearean Love Hat." It causes him to speak proper Shakespearean when wearing it and also makes Iris & Misery fall for him.