On Halloween, Paul Friday trades his psychologist's gravitas for a monster mask, dresses in black and waits quietly in the attic of his Oakmont garage. Best of all, the chief of clinical psychology at UPMC Shadyside lives in the last house on his street.

Perched atop the garage is a big ugly pumpkin with a sheet waving in the wind. Trick-or-treaters must step into the garage, look up the stairs of a pull-down ladder and tell the monster how many people want candy. A bucket full of chocolate bars, suspended from a rope, bangs down the ladder.

After the last person takes a piece of candy, "that's when the fire extinguisher goes off. The kids scream and run off. I always end up with more candy at the end of the night than I started with," Dr. Friday said, laughing. "I pick up all the candy the kids throw. I go back up into the attic and I sit there. I drink a beer and read the paper."

Dressing up as a monster, Dr. Friday says, allows him to "express my dark side without guilt."

He added that "masks probably reveal more than they hide."

Typically, women choose Halloween costumes such as beauty queens, princesses, animals or food while men gravitate toward warriors, villains and agents of death such as Darth Vader, Freddy Krueger and Leatherface, star of "The Texas Chainsaw Massacre."

Halloween is "the day of the year when you're allowed to let go" but the Mars/Venus dynamic played out between genders still holds, he said.

That's because, "Women will tend to pick costumes that maintain their gender. Women tend to hold on to almost everything, and guys want to let go," Dr. Friday said.

"Women tend to do foodstuffs much more than men. You can see through the costume. You recognize immediately who the person is. You see their head and you know who it is. Guys, on the other hand, cover their identity more than women do," he said.

People who dress as fairies or princesses are revealing something about themselves because these roles "represent one's lost innocence or beauty or a return to a safer or simpler time."

In Western civilization, he continued, "We line up gender expectations. The guys are allowed to be the bad boys, and the girls are going to hold on to some propriety."

Laura Prieto, professor of women's studies and history at Simmons College in Boston, agrees that your choice of a Halloween costume is a way to express a part of yourself that is submerged.

"I think that some of that comes into play today, much more for adults than children," she said, recalling a college student who won a costume award by appearing at a party as a halogen lamp that lit up. The halogen lamp, she added, is practically a universal symbol of college life.

In another creative twist, a straight man dressed as Queen Amidala, the royal leader of Naboo. He was, Dr. Prieto said, poking fun at this female character played by Natalie Portman in the "Star Wars" movies.

Gays and lesbians have their reasons for dressing up, too, she said.

"Some cities, such as Philadelphia and Toronto, have had pretty sizable drag parades staged by the local gay community. They are asserting their right to public space and being visible in a way that's less controversial than wanting to march in a St. Patrick's Day parade. They can make that public political statement with a sense of fun that people are more able to accept on Halloween.

"It's a world-turned-upside-down kind of day," Dr. Prieto said.

Dan Santoro, a sociology professor at the University of Pittsburgh's Johnstown campus, believes people enjoy Halloween because it's a chance to revel in fantasy.

"We live in this mundane world. We go to work. It gives us a chance to step out of those serious personas we have to put up every day. It's almost like Mardi Gras in that regard. People can escape from what they are rather than be what they are," Dr. Santoro said.