Why do people enjoy horror movies?

I'm just back from watching The Ruins. These days when I go to the theatre to watch a horror movie, I no longer concentrate on the scenes intended to cause the most trauma. I spend my time looking around, observing how people react.

In Doomsday, when a man is being burnt alive to feed a horde of post-apocalyptic cannibals, the audience is cheering. It's supposedly cool when the Jigsaw Killer in the Saw movies succeeds in making people fall into his gory traps, and people are also cheering when they watch a man being amputated with a hunting knife in The Ruins.

What's so funny about these scenes? I watched carefully to make sure there weren't any hidden directorial tricks involved -- some sort of comic undertone or the likes. Were the actors smirking when they were performing these horrifying scenes? Did the background music turn funny? Were the victims in these movies acting in a comic manner?

No. Nothing, nil, nada. All these extremely repulsive and horrifying scenes weren't meant to be funny. They were meant to be scary, but most people laugh at them. I look at the gleaming faces in the dark -- teenagers, middle aged people, single moms, kids, seniors. Why?

And, from a critical point of view, how successful are these scenes really as artistic devices/constructions? Is the audience really laughing at the scenes or the artists involved?

According to a Science Daily article, researchers at Berkeley have concluded that people love horror movies for the simple reason that they enjoy being scared. The audience perceives that there is no real threat, and even the acting which is meant to evoke feelings of trauma and shock is, bluntly put, an articulate hoax, and therefore enjoy them.

But it still doesn't explain anything beyond the surface layer. A hundred years back, Dracula in a silent movie would absolutely terrify the living shit out of anybody. When did we change? Have we gotten so used to movies, that we unconsciously refuse to immerse in it, but still appreciate it to satisfy a primal instinct deep inside our genes?

We're laughing at something scary because we know it's not real. But violent movies do breed violent acts. What is real, then? And -- how will violence be portrayed in future?

Suddenly, I'm very genuinely scared.