Critics and audiences alike reviled The Devil Inside. Reviews for the weekend's new release, about a woman who performs unauthorized exorcisms, were abysmal at best, while audiences gave it an extremely rare "F" CinemaScore grade after screenings. Yet, in what's being called a "box office shocker," The Devil Inside won the weekend, knocking blockbuster Mission: Impossible ā€” Ghost Protocol from its perch. The low-budget horror flick earned $34.5 million, one of the biggest January openings ever. How did this star-less film overcome its dreadful reviews? Here, four theories:

1. Young moviegoers turned out
Credit a young crowd for propelling The Devil Inside to the top, says Amy Kaufman at the Los Angeles Times. About 85 percent of the film's audience was under the age of 35, while 60 percent was under age 25. It's a crowd that's had little to attract them to cineplexes over the past several weeks, says Pamela McClintock at The Hollywood Reporter, with family-friendly fare, like Alvin and the Chipmunks and We Bought a Zoo, and older-skewing awards bait, like The Girl With the Dragon Tattoo and Tinker, Tailor, Soldier, Spy dominating new releases.

2. Its marketing strategy took a page out of the Paranormal Activity book
The Devil Inside was released by Paramount's micro-budgeted Insurge label, which was launched to recreate the mammoth success of the low-budget horror film Paranormal Activity, says McClintock. The 2009 movie and its two sequels have earned nearly $300 million combined in U.S. box office on shoestring budgets and Insurge borrowed the Paranormal marketing strategy to sell Devil, says Grady Smith at Entertainment Weekly. The TV spots spliced the film's grainy "found footage" horror scenes with shots of audiences in a theater screaming at the screen. This likely played a huge role in selling tickets.

3. The genre is extremely popular
Demonic tales and "possession movies" have been trending up in recent years, notes Smith. The genre's resurgence began with 2005's The Exorcism of Emily Rose, which grossed an impressive $75.1 million on a modest budget. Since then, films including 2009's A Haunting in Connecticut ($55.4 million), 2010's The Last Exorcism ($41 million), and 2011's Insidious ($54 million) all outperformed industry expectations. "Thanks to their tiny budgets" ā€” Insidious cost just $1.5 million to make ā€” these films have been particularly lucrative. The success of The Devil Inside will likely perpetuate the trend.

4. It was the weekend's only new release
"Being the only new movie in wide release certainly helped" The Devil Inside take the box office crown, Don Harris, Paramount's head of distribution, tells The Wrap. "When we saw that date become availableā€¦ we jumped on it." Furthermore, the first weekend in January has been traditionally lucrative for "non-family movies." In 2009, The Unborn scared up $19.8 million on the same weekend, says McClintock, while Daybreakers opened to $15.1 million in 2010.