“Blood-curdling horror” isn’t just a cliche. According to a recent study conducted by the BMJ (formerly known as the British Medical Journal), it’s a fact: People who watch horror movies have a greater amount of coagulant (specifically coagulant factor VIII) in their blood.
In this trial, 24 healthy volunteers, 30 years old or younger, were asked to watch the 2010 movie horror movie Insidious, as well as a 2014 documentary, A Year in Champagne. Some of the participants watched the documentary first, while others started with the horror. Their blood was taken before and after their movie-going experience.
The results? The participants had greater levels of coagulant factor VIII in their blood after watching the horror movie.
According to the study,
The mean baseline levels of coagulant factor VIII of the remaining 21 participants were similar before both the horror movie and the educational movie (mean difference −2.9 IU/dL, 95% confidence interval −10.1 to 4.2). However, the mean change in levels (difference between levels before and after the movie) was higher after the horror movie than after the educational movie (mean difference 11.1 IU/dL, 1.2 to 21.0, fig 2⇓). Coagulant factor VIII levels increased in 12 (57%) participants during the horror movie, but only in 3 (14%) during the educational movie. Levels decreased in 18 (86%) participants during the educational movie, but only in 9 (43%) during the horror movie.
The researchers also described a secondary outcome in the volunteers: They all reported fear.
This isn't the first time scientists have studied horror movies and their effects on viewers. Researchers have learned that horror movies help dieters lose weight and may create an urge to smoke.