Science can be scary. Flickr via Creative Commons/joebeone

As you don your costume and ready the candy bowl for tonight, you might be wondering just how much truth there is behind some of the spookiest Halloween traditions. While many ghouls and ghastlies belong to the realm of myth, there are still some scary stories that have a grain of truth to them.

Can You Die Of Fright?

Short answer: yes. Shock and stress can flood your system with adrenaline and stop your heart -- it’s sometimes called “stress cardiomyopathy syndrome.” When your sympathetic nervous system lights up and kicks your metabolism into high gear, it increases your heart rate, breathing rate, and makes you sweat more. Some doctors suspect that the fright-induced adrenaline rush affects small blood vessels leading to the heart, and also seems to have the potential to stun your heart muscle. Younger people probably won’t drop dead from a good scare, though; dying of fright seems to be much more likely in older, post-menopausal women with lower levels of the hormone estrogen, which protects the heart.

Fear and superstition can also make people with heart problems more likely to die at certain times of the year. A 2001 paper in the British Medical Journal found that Japanese-Americans and Chinese-Americans are more likely to die from heart problems on the fourth of the month; in Japanese and Chinese culture, the number four is associated with death. When researchers looked at a population of white Americans over the same time period, they found no such mortality peak around the fourth. The scientists dubbed this fourth-day mortality peak the “Baskerville effect,” after the character in the Sherlock Holmes novel “The Hound of the Baskervilles” who frightens himself to death with visions of a demonic dog.

Can Your Hair Turn White From Fear?

Short answer: not exactly, but kind of. The hair that comes out of your head is dead tissue, so it can’t actually change color. A massive fright won’t turn you frosty all over. However, there is an autoimmune condition called alopecia areata where a person’s pigmented hairs will fall out, and only leave gray or white hairs behind. And stress could trigger an attack.

“It’s conceivable for a person who has a tendency for alopecia areata to go through a stressful experience which makes it flair up and the first thing that happens is their dark hair falls out,” dermatologist David Orentreich told NBC News in 2009. “And that can happen quickly -- in days or weeks -- leaving just the gray hair.”

Do Vampire Bats Want To Suck My Blood?

We’d love to assure you that there’s no chance of getting bitten by a vampire bat, but there have been numerous reports of the critters sinking their teeth into people, particularly in Peru. Take heart in the knowledge that they’re much more likely to go for some tasty cow blood, though. Researchers think vampire bats may be more likely to snack on humans as their rainforest habitat is cut down.

Thankfully, there’s little chance of a vampire bat sucking you dry -- a typical bat will drink only around an ounce of blood in a meal, while the average person has about 160 ounces of blood in the body. The greater danger is from rabies -- while only a small fraction of bats carry the deadly virus, it can jump from bat to human. In 2010, Peru’s health ministry sent workers out to vaccinate more than 500 people bitten by vampire bats following the deaths of at least four children in an indigenous Amazonian tribe.

However, there may actually be a silver lining to vampire bat bites: some people in the Amazon that have encountered the little bloodsuckers are developing natural antibodies to the rabies virus.

Do Creeps Actually Poison Halloween Candy?

Most of the stories about creepy people poisoning Halloween candies are urban legends, but there have been a few scattered cases of tainted treats. The most gruesome poisoners, however, aren’t shadowy strangers, but family members. On Halloween night of 1974, Texas man Ronald Clark O’Bryan killed his own eight-year-old son with cyanide-laced Pixy Stix in order to collect on a life insurance policy. O’Bryan was executed by the state in March of 1984.

Why Is It So Hard For People To Shoot Zombie Family Members?

There’s always some scene in zombie movies where a gun-toting hero hesitates to kill a newly zombified friend or family member -- sometimes with fatal consequences. Why doesn’t our sense of self-preservation kick in, especially when our loved one is obviously rotting away and/or itching to chomp on our brains?

As psychologist Melanie Tannenbaum explains in a blog post at Scientific American, humans are primed to anthropomorphize things. Studies show we are remarkably quick to attribute human-like qualities to robots, so just imagine how hard it is to de-anthropomorphize a family member -- even one that’s starting to turn a little ripe.

“If you think now that it would be difficult to axe someone you once loved in the brain before they eat you alive, think about how much more difficult it would be if you were living in an unpredictable environment (as a post-apocalyptic world, almost by definition, must be) and therefore even more likely to ascribe human-like, thinking, feeling capacities to your now-undead friends and relatives,” Tannenbaum wrote.

If Ghosts Exist, How Many Are There?

If there really are spectral shades haunting the mortal plane, we probably wouldn’t be able to move without bumping into them. If we assume, for the moment, that everyone who dies becomes a ghost, then all we need to know is the total number of dead humans that have piled up throughout the ages. The Population Reference Bureau estimates that about 107.6 billion people have walked the Earth by the year 2011, at which time there were nearly 7 billion people alive on the planet. So, assume a ghost population of roughly 100 billion souls.

If we spread out the 100 billion ghosts equally distant from each other across the entire surface of the Earth, there would be about 507 ghosts per square mile -- roughly the population density of the Netherlands. But suppose the ghosts don’t like haunting the ocean. If they just stick to the land -- surface area of about 57 million square miles -- then there would be 1,754 ghosts per square mile, making the spook population almost as dense as Taiwan’s. If ghosts prefer to stick to habitable land (24.6 million square miles), there would be about 4,065 ghosts per square mile, almost exactly the same kind of population density you’d find in San Diego, Calif.