Can Fear Really Be 'Bloodcurdling'?

Researchers in the Netherlands have found that fear and horror movies can increase levels of a blood-clotting protein. Ina Fassbender/REUTERS

The phrase bloodcurdling scream comes up often in detective novels. Now researchers say that fear actually can increase activity for a blood clotting protein.

Researchers at the Leiden University Medical Center in the Netherlands asked participants in a recent study to watch two movies: a horror film and an "educational" film. After each one, the researchers took blood samples. The results were published Wednesday in The BMJ's annual quirky-but-true Christmas issue.

The 24 participants, all 30 years old or younger, watched Insidious (a scary movie from 2010 in which evil spirits mess with a child) and A Year in Champagne (a documentary from 2014 about how champagne is made). Fourteen of them watched the horror movie first; the other 10 began with the educational film. Participants waited a week between films and viewed them at the same time of day and in the same place (a makeshift movie theater in a university meeting room).

"To reduce the risk of confounding by superstition, no movies were shown during a full moon or on Friday the 13th," the researchers write in the study.

Participants rated the films on a scale of 0 to 10 for how scary they were. The mean difference between those ratings was 5.4. (Complex rated Insidious one of the scariest movies of all time.)

For the majority of participants (57 percent), the researchers found that levels of factor VIII, a blood-clotting protein, had increased during the horror movie. For the vast majority of participants (86 percent), factor VIII levels dropped while watching the educational movie.

The researchers believe their study is the first time anyone has extensively explored the science behind the phrase bloodcurdling, which they note also exists in other languages—"à vous glacer le sang" in France and "bloedstollend" in the researchers' native Netherlands. Previous studies had examined the connection between physical stress and blood coagulation, but not acute fear, they say.

There is a biological reason why fear can affect blood-clotting activity, the researchers say: "This poses an important evolutionary benefit, by preparing the body for blood loss during life threatening situations."

Perhaps next the researchers will see if anger really can make the blood boil.