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British Medical Journal

Clinical Articles, Lead Article

Scary movies can curdle blood

Watching horror, or “bloodcurdling,” movies is associated with an increase in the clotting protein, blood coagulant factor VIII, according to small study conducted in Amsterdam, and published in the BMJ Christmas issue.1

The results suggest that using the term “bloodcurdling” to describe feeling extreme fear is justified, say the researchers.
The term dates back to medieval times and is based on the concept that fear or horror would “run the blood cold” or “curdle” (congeal) blood, but the validity of this theory has never been studied.
So researchers in The Netherlands set out to assess whether acute fear can curdle blood, which they say poses an important evolutionary benefit, by preparing the body for blood loss during life threatening situations.

The study involved 24 healthy volunteers aged 30 years or younger recruited among students, alumni, and employees of the Leiden University Medical Center. 14 were assigned to watch a frightening (horror) movie followed by a non-threatening (educational) movie and 10 to watch the movies in reverse order.
The movies were viewed more than a week apart at the same time of day and in a comfortable and relaxed environment. Both lasted approximately 90 minutes.

Before and after each movie (within 15 minutes), blood samples were taken and analysed for markers or “fear factors” of clotting activity. After each movie, participants also rated the fear they experienced using a visual analogue fear scale ranging from 0 (no fear at all) to 10 (worst fear imaginable).
Participants also reported whether they had already seen the movie and completed a general questionnaire on lifestyle and favourite movie genre.
The horror movie was perceived to be more frightening than the educational movie, with a 5.4 mean difference in fear rating scores.

The difference in coagulant factor VIII levels before and after watching the movies was higher for the horror movie than for the educational movie.
Levels increased in 12 (57%) participants during the horror movie, but only in three (14%) during the educational movie. Levels decreased in 18 (86%) participants during the educational movie, but only in nine (43%) during the horror movie.

However, the researchers found no effect of either movie on levels of other clot-forming proteins, suggesting that although coagulation is triggered by acute fear, this does not lead to actual clot formation.
They point out some study limitations, but conclude that, in young and healthy adults, “watching bloodcurdling movies is associated with an increase in blood coagulant factor VIII without actual thrombin formation.”

BJC Arrhythmia Watch interviewed the lead study author, Dr Banne Nemeth, Department of Clinical Epidemiology, Leiden University Medical Centre.

1. Are there any possible/potential clinical adverse effects associated with the observed increases in levels of Factor VIII?

The mean increase in factor VIII levels of 11.1 IU/dL associated with acute fear could be clinically relevant, as every 10 IU/dL increase in coagulant factor VIII levels is associated with a 17% (95% confidence interval 7% to 28%) increase in the risk of venous thrombosis. However, no actual thrombin formation was observed, thus in these healthy volunteers watching horror movies seemed safe in terms of thrombosis risk.

2. Did you measure any hormones such as cortisol or norephinephrine, which are associated with stress? If not, any plans to do so in another study?

No, we did not. After each movie the participants completed a visual analogue fear scale, designed specifically for this study. This scale estimates the degree of fear experienced while watching a movie, ranging from 0 (no fear at all) to 10 (worst fear imaginable). This scale was used to determine whether participants find the movie scary or not.

3. Is there any clinical message from the study e.g. should patients with heart problems avoid scary movies?

We did not study this. The fear factor trial was conducted with the help of young and healthy volunteers. In these persons no actual thrombin (blood clot) formation was present, for them it was safe to watch scary movies. So far there is no evidence that patients with heart problems or other medical conditions cannot watch scary movies.

4. Did the investigators consider Insidious to be genuinely scary?

Yes, we consulted a “horrorology” expert to choose the scary movie. The aim was to pick a movie with a consistent pattern of fear. Also the participants perceived the horror movie as being frightening, with a mean score on the visual analogue fear scale of 5.4 (compared with corresponding values for the educational movie 0.0.


1. Bloodcurdling movies and measures of coagulation: Fear Factor crossover trial. BMJ 2015;351:h6367.

Published on: January 14, 2016

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