The term "bloodcurdling" has been around since at least the early 19th century, used to refer to something that induces fear. Of course, no one ever actually thought your blood could curdle or coagulate from fear, but it seems there's at least a grain of truth in the term after all.
According to research conducted by a team at the Leiden University Medical Centre in the Netherlands and published earlier this week, watching horror films does actually cause an increase in a particular blood coagulant factor responsible for blood clotting.
The study was conducted on 24 healthy participants aged 30 or younger. These participants were divided into 2 groups, one of 10 participants, the other of 14. The group of 10 was assigned to watch a non-scary educational film followed by a horror film, and the group of 14 was assigned to watch the horror film followed by the educational film, viewed a week apart.
Before and after viewing each film, blood samples were taken from each participant. These samples were analysed for clotting activity, revealing an increase in coagulant factor VIII in more participants that viewed the horror film than watched the educational film.
"Levels increased in 12 (57 percent) participants during the horror movie, but only in 3 (14 percent) during the educational movie," the team wrote. "Levels decreased in 18 (86 percent) participants during the educational movie, but only in 9 (43 percent) during the horror movie."
This didn't result in actual clotting. Although there was an increase in one coagulant factor, there wasn't a corresponding increase in the proteins required for clotting to take place. So while the coagulation process can be kicked off from intense fear, it doesn't lead to the blood actually clotting in the veins.