"The Ring" is a great way to trigger this "bloodcurdling" effect. IMDB

Bloodcurdling.

That underused adjective is one of the classic ways to describe the tense and terrifying feeling that an excellent horror film provides.

But what is it about fear that led to the creation of word that combines blood with "curdle," a word referring to the solidification of liquid?

Could fear actually curdle blood?

In a way, yes, according to a group of medical researchers from the Netherlands. Or at least, it can make blood more likely to clot, which is what happens when a wound congeals and stops bleeding.

The researchers studied the effects of horror films on the blood of a group of volunteers and found that watching a scary flick actually caused a significant increase in levels of blood coagulant factor VIII, a clotting agent.

"We hypothesised that acute fear activates the coagulation system and that this poses an important evolutionary benefit, by preparing the body for blood loss during life threatening situations," the authors write in their study, which was published in the December 16 issue of the BMJ — the prestigious medical journal's light-hearted "Christmas Issue," which uses science to tackle important and somewhat whimsical questions like "are British people's teeth really that bad?"

In order to test out their bloodcurdling theory, they recruited 24 volunteers and split them into two groups, one of 14 and one of 10.

They took blood samples from each group and sat them down to watch a movie. The researchers had asked the volunteers, who had no idea what they would see, to avoid alcohol and tobacco during the day.

The 14 first saw the 2010 film "Insidious," which the researchers describe as "a very frightening movie" in a video accompanying their study.

The other group saw the documentary "A Year in Champagne." It's "a very dull educational movie," according to the Dutch scientists.

They took another blood sample from each group after the film.

One week later, they called the groups together again to take another blood sample, watch the film they hadn't seen before, and to provide yet again more blood.

The results

Study participants rated how scary the films were on a "fear scale" measuring from 0-10. Unsurprisingly, "A Year in Champagne" averaged around a zero, while "Insidious" was approximately a seven.

But it's the analysis of the blood that holds the clues to what that fear can do.

Two participants' samples couldn't be measured because they were "visibly haemolytic" — damaged, which would change the readings. One other participant had to be excluded since fear about getting his blood drawn caused him to "consume a family pack of chocolates" before the second drawing in an attempt to relax (he fainted during the process anyway). Eating changes blood levels, so they couldn't count his.

For the rest of the volunteers, that specific clotting factor was at a similar baseline before both films.

But it spiked in 12 of the 21 after watching the horror film, while going up in only three after "A Year in Champagne." And after watching the boring film, clotting factor levels in blood actually went down in 18 participants, while only 9 saw a decrease after "Insidious."

The final findings indicate that a scary film did indeed cause an increase in one blood clotting factor (though not in others). That validates their hypothesis: that fear prepares us, physically, for danger. But that alone can't tell us why we react to horror films in the way we do.

And while this is a very small sample — smaller than the researchers had estimated would be necessary for truly conclusive results, it does point to a deep authenticity in the meaning of "bloodcurdling."

After all, it's a word that, as the authors write, dates back to the Middle Ages, when people wrote of fear that could "run the blood cold," like the old German phrase "das blut in den Adern erstarrt," or "freezes the blood in your veins."