Science has the answer. According to a recent article, "Horror film gene that makes some scream while others laugh", it's a matter of brain chemistry. The COMT gene weakens our ability to control our emotions: the more copies of the gene you have, the less your restraint, and the more affected you are by unpleasant pictures. In the study, participants with just one COMT gene (which is about half the population) "were able to keep their emotions in check far more readily", while just one COMT gene predisposed viewers to be "significantly more startled by frightening images than others."

The article doesn't live up to its headline before closing by saying other variables influence the situation — which seems obvious to me, and not on a neurochemical level. The horror genre features ample entrails and other viscera, and some moviemakers mistakenly use this visual device as a substitute for plot, tension, character development, and depth. As a result, we're presented with elementary storytelling awash in senseless violence, all masquerading as a horror film. For some people, likely reactions to such cinematic sludge are boredom or nausea; for others, it's laughter. The film may not have been designed to be a comedy, but it inadvertently is, and we can't help but derisively observe by how far the filmmakers missed their target.

Need proof? The Internet Movie Database classifies Manos: The Hands of Fate as horror. This representative of the genre is #9 on the IMDb's Worst 100 Films, voted there as a result of its popularity from being featured on Mystery Science Theater 3000. Anyone who's ever seen Manos knows there's nothing to fear here.

Good comedy is hard to do; so is good horror. A failure at one can result in a success at the other — so long as you have the genes to appreciate it.