In just four years, the Paranormal Activity franchise has earned over $650 million at the box office.

Facebook/Paranormal Activity

f the box-office success of Paranormal Activity 4 is any indication, the horror franchise will continue to haunt movie theaters for the foreseeable future. Though its overall gross was down from its predecessor, last weekend's Paranormal Activity 4 was the No. 1 film at the box office. Paramount, the studio behind the franchise, is already working on the all-but-inevitable Paranormal Activity 5, as well as producing a spin-off of the series — teased in a post-credits sequence at the end of Paranormal Activity 4 — which will be aimed at the Latino market, reports Deadline. In just three years, the four Paranormal Activity films have earned over $650 million at the worldwide box office, with each entry's production budget costing less than $10 million. How did the Paranormal Activity franchise become such a surefire hit for Paramount? Here, three theories:

1. It aims low, and hits the mark
Paranormal Activity 4 "gives you exactly what you'd expect and nothing more," says Anothony O'Connor at Film Ink. The franchise has never had any pretensions about its aspirations: Each entry is expressly designed to serve as a "a date movie for teenagers who scream whenever something moves in front of the camera, or there is a sudden, loud noise." As long as the franchise continues to make good on its modest promises, its intended audience will continue to catch it in theaters.

2. Horror fans don't care about bad reviews
Paranormal Activity 4 may have scored a dismal 27 percent positive reviews, says Jeremy Kay at The Guardian, but "the musings of critics mean little to genre fans," who routinely attend horror films even when they've been scathingly reviewed. The franchise's loyal fans "were champing at the bit" to see the series' increasingly-complex mythology continue to expand. 

3. The films are extremely popular abroad
Though Paranormal Activity 4 slightly underperformed in the United States, its opening gross in the foreign market was even better than both Paranormal Activity 2 and 3, says Frank Segers at The Hollywood Reporter. As long as these movies continue to emerge as the "number one attraction on the foreign theatrical circuit," loyal fans can anticipate — and critics can dread — more entries in the found-footage horror franchise.