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What do the greatest horror movies all have in common?

They all have memorable soundtracks that elicit an instantaneous sensation of terror. Indeed, if you watch the films without any sound, they become a great deal less frightening.

But what is it about the music that makes it terrifying? More specifically, if sounds are just oscillations in the air, what is it about our biology that makes us respond with fear?

The Fear Response

In terms of evolutionary biology, there’s an evident survival advantage to the immediate detection of a threatening situation.

Thinking is time consuming, especially when you’re staring a ravenous lion in the face. When every second counts, you don’t have the time to stop and process the information deliberately.

Considering it takes longer to process and contemplate visual information, the animal brain is wired to react to quicker sound-processing mechanisms—a characteristic that provides survival advantage and has been selected for in the wild.

And that’s exactly what we discover in nature: several vertebrates—humans included—generate and react to harsh, nonlinear sounds and vocalizations when frightened. This generates a nearly instantaneous feeling of fear or anxiety.

But what is it about nonlinear sound that makes it frightening?

When an animal screams, it produces a scratchy, irregular sound that stretches the capacity of the vocal cords past their typical range.

Our brains have evolved to identify the properties of nonlinear sound as unpleasant and suggestive of hazardous situations.

The interesting thing is, we can artificially imitate a wide array of these nonlinear sounds to get the same immediate fear response in humans.

And so, what was once an effective biological adaptation in the wild has now been co-opted by the movie industry to produce scarier films.

Music and Fear

We all are familiar with the shower scene from the classic film Psycho, and it’s probably one of the most terrifying scenes in the history of film.

But if you watch the scene without sound, it loses most of its affect. It’s only once you add back in the high-pitched screeching and bone-chilling staccato music that the fear response becomes thoroughly engaged.

To confirm our instinctive aversion to this nonlinear sound, UCLA evolutionary biologist Daniel Blumstein carried out a study examining the emotional reactions to two types of music.

Study participants listened to a collection of emotionally neutral scores and scores that contained nonlinear elements.

As anticipated, the music with nonlinear elements elicited the strongest emotional reactions and negative feelings. This response is simply part of our anatomy and physiology.

Whether Hollywood understands this physiology or not, it knows intuitively that the use of nonlinear disharmonious sound is still the best way to get a rise out of the viewers.

Want to see the fear response in action?

Check out these 10 Essential Horror Movie Scores.