An essential part of any horror flick is the bloodcurdling scream that lets you know things just got real. But while people have associated the word “bloodcurdling” with fear since medieval times, we’ve actually have had no idea scary stuff makes our blood, well, curdle.
Metaphors must gravitate towards the truth: A team of Dutch researchers found that fear increases protein factor VII, which is a part of the blood clotting process. It comes down to evolution; a threatening situation means there’s a chance of injury, meaning there’s a risk of blood loss. When you’re afraid, your blood coagulates faster so you run less of a risk of fatal bleeding.
“The term dates back to medieval times and is based on the concept that fear or horror would ‘run the blood cold’ or ‘curdle’ blood,” writes the researchers in their paper published in the British Medical Journal. “The validity of this theory has, however, never been studied.”
The team recruited 24 healthy volunteers under the age of 30 and separated them into two groups. The first group watched the horror film Insidious and then a week later the more low key documentary A Year in Champagne — the other group watched the same movies but in reverse. Each group had a blood sample taken before the first movie, two samples taken between the films, and then a final sample after the second movie.
Participants were asked to avoid alcohol and tobacco on movie days, and to avoid any undue connections the movies were purposefully not shown on a full moon or Friday the 13th. One participant had to opt out of the study because, in an attempt to ease his nerves for having his blood drawn, ate a family pack of chocolates and then fainted anyways.
The researchers found that coagulant factor VIII levels increased in 57 percent of the participants who watched the horror film, and in only 14 percent of those who watched the documentary. Of those who watched the horror film first, 86% of the participants had lower levels of the protein when they watched the documentary.
While the researchers believe their work proves that the word ‘bloodcurdling’ in a frightful context is now justified, the underlying biological mechanism that causes the process to happen is still unknown. In a press release, co-author Dr. Banne Nemeth emphasized that while fear increases the blood-clotting factors within the blood, it doesn’t mean that horror films actually increase your chance of developing blood clots.
Still, in a cheeky nod to the annual BMJ Christmas edition, the authors were compelled to offer one bit of advice in a sign-off nerd joke: “A truly relaxing and merry Christmas, without exposure to frightening situations, seems to be advisable to prevent venous thrombosis.”