Anyone who has seen a movie can identify with a certain reaction to a horribly bloody or gross scene: Gross. But. Can't. Look. Away. A new study in the Journal of Communication tries to get at exactly what's going on here by analyzing viewers' responses to "body envelope violations/death and body products." That is, blood and poop.
The study is available for free online, so check it out if you want the details about how different types of disgusting scenes affected viewers' physiological responses and ability to remember what they were seeing. To me, the overall message here is what's most interesting. As the researchers write: "Disgust ... makes us feel bad — but it has functionally evolved over time to compel our attention, thus making it a quality of entertainment messages that may keep audiences engrossed and engaged."
Evolutionary psychology — the idea of connecting aspects of modern human behavior to our evolutionary roots — is a controversial one in some quarters, and in many cases it can be used to over-explain certain behaviors and falsely justify others. ("Men have to be on the prowl, baby! We can't settle down. Evolution says so.")
But here, it makes sense. Back in the day, if one of our ancestors came across a disgusting scene, it was a very important one to pay attention to from a survival standpoint (corpses and blood and poop can all kill you — especially when you don't have antibiotics and other modern medical advances). So the adaptive response would have been (1) Don't go near it!, and (2) Don't ignore it, either — remember this and avoid it in the future. That's a nice explanation for some of the push-pull going on when we sit entranced and horrified as Jason does his thing.