On any other day of the year, a French maid is just a French maid. Batman, just Batman. But on Halloween – she’s sexually repressed and flaunting her inner vixen. He clings to fantasies about being an all-powerful superhero. Indeed, experts say our Halloween costumes often aren’t random choices. They reveal hidden personality traits, reflecting our inner urges on the one day it’s OK to abandon societal rules and regulations. “It’s an opportunity to express things we’re normally not allowed to express,” says G. Dennis Rains, a psychology professor at Kutztown University in Kutztown, Pa. “It’s permission to let your underside or dark side come out.” Consider psychologists’ interpretations of these 10 popular costumes.