Horror movies really do curdle the blood - and could be bad for health, warn scientists
Fear prepares the body for blood loss during life threatening situations, say the Dutch researchers
A study found watching scary films boosts a blood clotting protein known as Factor VIII.
The research published in the Christmas issue of The BMJ suggested using the term 'bloodcurdling' to describe feeling extreme fear is justified.
But it also has a serious side as there could be clinical implications - with the possibility of 18 certificate flicks raising the risk of a blood clot by about a fifth.
The term 'bloodcurdling' dates back to medieval times and is based on the concept that fear or horror would 'run the blood cold' or 'curdle', or congeal, blood. It is the first time that scientists have looked to see whether there was any truth in the saying.
The Dutch researchers who set out to assess whether acute fear can curdle blood say it poses an important evolutionary benefit - by preparing the body for blood loss during life threatening situations.
They recruited 24 healthy volunteers aged 30 or younger - all students, alumni and employees of the Leiden University Medical Centre - with 14 assigned to watch a frightening film followed by a non-threatening educational movie and the others viewing them in reverse order.
Watching the 2010 supernatural horror film 'Insidious' starring Rose Byrne raised Factor VIII levels by an average of over 11 IU/dl (International Unit/decilitre) - a reading that has been linked with an increased risk of a venous thrombosis, or blood clot.
Levels of the clotting agent increased in 12 (57 per cent) of participants during the horror movie - but only in 3 (14 per cent) during the educational movie 'A Year In Champagne.'
Levels decreased in 18 (86 per cent) participants during the educational movie but only in 9 (43 per cent) during the horror movie.
Dr Banne Nemeth said: "We found watching horror, or 'bloodcurdling', movies was associated with an increase in blood coagulant 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 per cent increase in the risk of venous thrombosis."
The movies were viewed more than a week apart at the same time of day and in a comfortable and relaxed environment. Both lasted about 90 minutes.
Before and after each movie blood samples were taken and analysed for markers or 'fear factors' of clotting activity.
After each 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 average difference in fear rating scores.
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.
Dr Nemeth added: "The underlying biological mechanism of acute fear associated with an increase in coagulation activity is still to be unravelled.
"Although it's not immediately obvious by which means our results could confer clinical benefits a broader implication of these study results is that after centuries the term 'bloodcurdling' in literature is justified.
"In young and healthy adults, watching bloodcurdling movies is associated with an increase in blood coagulant factor VIII without actual thrombin formation."