Why screams are scary

Leave it to a group of new parents to be inspired to study the effects of screaming on the human brain.

David Poeppel, a professor of psychology and neuroscience at New York University and director of the Max Planck Institute's Department of Neuroscience in Frankfurt, Germany, normally studies speech and communication. Recently, however, he found himself sharing a laboratory with a group of new parents in New York.

“[They were] in the middle of either having babies, just had babies, or obsessing about how their brains are being hijacked by the screams in their lives,” Poeppel says.

Poeppel and his colleagues know, as people in every culture instinctually know, that screams have a disturbing effect on the human brain. What they didn’t know was why screaming, as opposed to loud singing, or loud laughter, causes humans to feel frightened or alarmed.

In order to figure out how screams work, the scientists began collecting screams. First they looked online, downloading screams from movies.

“Then we had people come into the lab and scared the hell out of them and recorded screams in the sound booth. This was done in the NYU community, and there are some surprisingly good screamers. Just bloodcurdling, scary stuff,” Poeppel says.

After they had amassed a database of screams, Poeppel and his colleagues applied acoustic analysis to the recordings. They ranked the screams on a scale, according to what people thought was most frightening.

What they found was that, when it comes to the frightening quality of a scream, everything depends on an attribute known as “roughness.”

“Roughness means that the sound is modulated in its loudness, or its amplitude, very rapidly. Unlike speech, which of course also has louder and softer parts as we're having this conversation, those modulations are pretty short. Screams are modulated very rapidly, between 30 to 150 times per second, and it's that feature of rapid modulation that gives us the feeling of what's technically called roughness,” Poeppel says.

In other words, the more a particular scream contains the attribute of “roughness” or rapid modulation, the more it is perceived to be frightening.

The researchers also studied brain images to learn which parts of the brain were activated by different screams. They found that the more roughness modulation a scream contains, the more it activates the fear circuitry in the brain.

“It's a very effective stimulus to elicit a response and to get you to act,” Poeppel says.

The study, which was published last month in the journal Current Biology, only researched adult human screams, not baby screams. Poeppel, however, says he thinks the principle of roughness would apply to children’s screams as well.

“I think there are some screams that you hear from your own kids that you say well, you know, ‘Kid’s hungry again.’ There are some screams, and I suspect those are the ones with a high degree of roughness, that really make you jump up and run.”

This story first aired as an interivew on Science Friday with Ira Flatow.