October 30, 2015

The scare factor and human survival: The science behind physical signs of terror

by Bonnie Denmark

A bloodcurdling scream. Eyes wide. Mouth agape. Hands raised. Muscles tensed. Breathing accelerated. These classic signs of terror are not just box-office boosters for Hollywood horror flicks. Rather, they are part of the universal human survival toolkit.

Actress demonstrating initial reactions of fear and panic. (Bantosh, 2006: Wikimedia Commons)

The outward manifestations of fear are an instinctive response that can help us escape an immediate threat. Here’s how: Stretching the face into extreme expressions gets blood flowing to the brain for quicker decision-making. Opening the eyes wide increases our field of vision to better scan the landscape for predators. Breathing rapidly through the nose heightens our sense of smell. Throwing hands into position near the face poises us to defend or strike. Contracted muscles prepare us to spring into action.  And the scream? It’s nearly impossible to ignore that time-honored alarm that works its magic by successfully activating the fear center of the brain. Further, people identify the direction from which a noise emanates more quickly and accurately if the sound is a scream, according to neuroscientist David Poeppel of New York University.

What makes screams special? While screams are undoubtedly loud and high-pitched, so are many other sounds that don’t activate our fear response. Poeppel and a team of international colleagues looked into what makes screams of terror so effective. It turns out that screams share a unique acoustic property, an auditory feature called “roughness.” The researchers measured the rate of change in sound intensity in samples of normal speech, singing, and screams collected from horror movies, YouTube videos, and volunteer recordings. Using a sound analysis method called the modulation power spectrum (MPS), the team charted how quickly the volume of a sound changed over time. They found that the volume of normal human speech stays within a small range, changing less than 5 Hertz per second (1 Hertz = 1 cycle per second), while the volume of screams fluctuates widely, from 30 to 150 Hertz per second. These volume fluctuations give screams the quality of roughness, which is not found in speaking or singing. The findings, published in August, were consistent across different languages.

Screams of terror activate the amygdala, the “fear hub” of the brain. (Memory Loss and the Brain online newsletter, Rutgers University. Image is in the public domain.)

Study volunteers were asked to rate sound clips on how scary they were. Amplitude was hand-modulated in the samples to produce different degrees of roughness. The participants rated the sounds that had rapidly-changing amplitude, i.e., those with greater roughness, as having a bigger scare factor. MRI brain scans of volunteers corroborated the findings: As participants listened to various sounds, it was only the screams that activated the amygdala, the structure in the brain that processes fear, whereas other loud and high-pitched sounds did not. And the rougher the scream, the greater the fear response in the listener, with the amygdala springing into action more vigorously to register danger. Among human sounds, only screams have roughness. However, alarm-makers have stumbled onto the secret, as burglar alarms and sirens also utilize this acoustic quality.

The biological response to screams works the same across cultures, language groups, genders, and ages. According to Harold Gouzoules, a professor of neuroscience and animal behavior at Emory University who was not involved in the study, “The ability to belt out a scream is deeply rooted in our evolutionary history, and is no doubt critical to our survival.” So before you dismiss on-screen scream queens (and kings), keep in mind that the look and sound of terror is a biological defense mechanism that helps humans survive in the face of extreme threat.



For more on how animals’ particular features help them survive as a species, read our Evolutionary Biology modules on Adaptation and Natural Selection.

Read an overview of the biology of fear- and anxiety-related behaviors (Steimer, 2002, Dialogues in Clinical Neuroscience).

Watch the video Stuff to Blow Your Mind: Screams of Terror – Science on the Web.


Written by

Bonnie Denmark holds an MA in linguistics and teacher certification in English, ESL, and Spanish. She has devoted her professional life to educational and accessibility issues as a computational linguist, multimedia curriculum developer, educator, and writer. She has also worked nationally and internationally as a language instructor, educational technology consultant, and teacher trainer. Bonnie joined the Visionlearning team as a literacy specialist in 2011, assisting the project by developing comprehension aids for science modules and creating other STEM learning materials.

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