A new study conducted by researchers in The Netherlands has found that the bloodcurdling effect of watching horror films is supported by an actual event in the human body involving the coagulation of the blood.
The term bloodcurdling is believed to have originated during the medieval period. It was based on the idea that the blood typically runs cold or "curdles" when an individual is exposed to frightening situations, but this theory has yet to be proven.
In a paper featured in the British Medical Journal's Christmas edition, researchers at The Netherlands' Leiden University examined whether acute fear does have the ability to cause the blood to curdle.
They believe that this trait provides a crucial evolutionary benefit in preparing the human body for the potential loss of blood during life threatening events.
The researchers asked 24 healthy members of the Leiden University Medical Center, including students, employees and alumni aged 30 years or younger, to participate in the experiment.
Fourteen of the participants were asked to watch a horror film followed by an educational film that was not threatening in any way, while ten of them were asked to watch the educational film first followed by the frightening one.
The participants watched the films more than a week apart in a relaxed and comfortable surrounding at the same time of day. Both of the features lasted for about 90 minutes.
The researchers collected samples of blood from the participants 15 minutes before and 15 minutes after watching the films. They then analyzed the samples for potential "fear factor" markers of blood clotting activity.
At the end of each film, the participants were asked to rate the level of fear they experienced through the use of a visual analogue scale, with the number zero denoting the absence of fear and 10 denoting the highest level of fear imaginable.
They were also told to report whether they have watched the film before as well as to answer a basic questionnaire regarding their lifestyle and favorite film genre.
The Leiden University scientists discovered that the participants considered the horror film to be scarier compared to the educational feature, with a mean difference of 5.4 in the group's fear rating scores.
The varying levels of coagulant factor VIII in the blood of the participants prior to watching the films and after they finished were also significantly higher for the frightening film than for the educational one.
Twelve members of the group, or 57 percent of the participants, had increased coagulant factor VIII levels while watching the horror film. Only three participants, or 14 percent, showed increased levels while watching the educational film.
On the other hand, 18 of the participants, or 86 percent, experienced decreased levels during the educational film, while nine of them, or 43 percent, had lower levels during the horror film.
Despite these findings, the researchers were not able to identify any activity in other proteins that could cause blood clotting. This suggests that even though acute fear can indeed trigger curdling (coagulation), the event does not cause the formation of an actual clot.
The team concludes that watching horror films can cause a rise in the coagulant factor VIII of the blood without actually forming thrombin, the enzyme that causes blood clotting, especially in healthy and young adults.
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