Ken Rogoff's favorite economics joke

here:

A drunk on his way home from a bar one night realizes that he has dropped his keys. He gets down on his hands and knees and starts groping around beneath a lamppost. A policeman asks what he’s doing.

“I lost my keys in the park,” says the drunk.

“Then why are you looking for them under the lamppost?” asks the puzzled cop.

“Because,” says the drunk, “that’s where the light is.”