Word Origins: Dunce Cap

A dunce cap is a cone-shaped paper hat formerly placed on the head of slow or lazy pupils. The dunce cap comes from a 13th-century philosopher named John Duns Scotus, who, not surpisingly, was born in Duns, Scotland.
This well-respected but terribly oblique scholar felt that conical hats actually increased learning potential. Here's the theory: knowledge is centralized at the apex and then funneled down into the mind of the wearer.
Scotus was an inveterate hair-splitter ( = a disputant who makes unreasonably fine distinctions) and came up with terms like "haecceitas," or "thisness." He was widely praised in his day, but eventually fell out of intellectual favour. His "duns cap" was a pretty obvious target of derision (contemptuous or jeering laughter; ridicule) and came to symbolize stupidity.
So the logic behind the dunce cap is that it makes slow pupils learn better, but it was later used to humiliate the wearer and motivate students to try harder.
Hats like this are never used now, but they sometimes appear in cartoons, often with a large letter "D" on them. A very similar practice on the European continent was a paper headdress known as donkey's ears, as a symbol of 'asinine' stupidity.