What Is Causing The Paranormal Activity In Paranormal Activity?

According to Movieline, Paranormal Activity only cost about $11,000 to make, which is an incredibly small budget for a movie. You could only buy two torturous Jay Leno puns with that kind of money. It's practically nothing. What's more, since putting an ad on Craigslist looking for people who have never seen a horror movie before to fill the screening audience was free, the trailer only cost however much it costs to rent a night vision camera from MTV's Fear.
"If it's not a ghost, what is it!?" Good question. What's causing the paranormal activity in Paranormal Activity? Here are a few possibilities:
—Zeus
—The producers of MTV's Fear.
—Risidual effects from Wendy Williams' "Diva Fan"