Scary movies, it turns out, can be good for us. In one experiment, a group of 32 healthy males and females (aged 20-26) either sat quietly in a room or were subjected to the film, "The Texas Chainsaw Massacre." Afterwards, the moviegoers showed higher levels of activated leukocytes - white blood cells (pictured), which are used by the immune system to defend against bodily invaders.

Horror films are also said to burn calories - but bear in mind, the study was co-commissioned by Amazon.com's LOVEFiLM. It was conducted by the University of Westminster, London. Watching "The Shining" brought on a calorie burn of 184, "Jaws" 161 calories, "The Exorcist" 158, and "Alien" 152 - but "The Texas Chainsaw Massacre" a mere 107.

We are attracted to horror movies because they are a kind of practice run for disastrously, dangerous scenarios. "[We] develop behavioral cognitive emotions that allow us to swiftly detect and appropriately respond to dangers," says horror researcher Mathias Clasen. Watch "The Shining," for example, and you're better prepared for that trip to your remote hotel.

Most of us come imprinted with good snake instincts, however. Virginia researchers asked preschoolers and adults to find a single target image in the midst of eight distracting images. Both children and adults detected snakes far more rapidly than flowers, frogs or caterpillars - it was the first time children had been tested for enhanced visual detection of evolutionarily relevant threat stimuli.

In another experiment, scientists wondered whether individuals mixing cocktails would be "peripherally blind" to a nearby spider. NOPE! A large percentage saw the creepy critter - all while taking comparatively little notice of things like houseflies, non-spider prototypes and syringes.

Feelings of fear and revulsion are meant to protect us. Yet human evolution hasn't kept up with modern civilization. Our deepest fears - spiders, snakes, heights, enclosed spaces - have woefully little in common with what actually kills us today: trans fats, cigarettes, cars, alcohol. While you might find the latter four at a Halloween party, they would never be strung up as "scary" decor.

Not everyone is affected by horror. Patient SM (not pictured) suffered amygdala lesions and showed no reaction to horror films, haunted houses or scary animals: "SM held the snake for over three minutes while displaying a wide range of exploratory behaviors: she rubbed its leathery scales, touched its flicking tongue, and closely watched its movements as it slithered through her hands."

How about our primate relatives? Are they freaked out by zombies? Results from Princeton University suggest they are. Macaques were shown unrealistic monkey faces as well as realistic ones. They looked longer at the realistic images, leading the researchers to think Macaques experience an "uncanny valley," which keeps them (and us for that matter) averse to disease and genetic deformities.

Did you know you smell differently after a horror film? Moviegoers in Vienna were shown a "fear film" (Candyman) and a "neutral film" and later had their armpits dabbed. The "lucky" women who were asked to sniff the swabs were able to distinguish between "the scent of fear" and non-fear. The stress hormone cortisol, however, was not the cause, implying fear pheromones have other origins.

Horror films can leave a permanent, negative impression. Seventy-five percent of students who were asked reported waking-life disturbances. Some examples: difficulty swimming after watching "Jaws," uneasiness around clowns, televisions and trees after watching "Poltergeist," a fear of camping and woods after watching "The Blair Witch Project," and anxiety when home alone due to the film "Scream."

But the science is very much out on the effects of horror video games. The genre is too new. Some studies have shown, however, that the intensity of the gaming experience does disrupt sleep. Australian researchers found gamers slept for shorter periods and that their sleep was less efficient.