You know how babies cry a lot? Yeah, that's a myth. Children and adults cry. Technically, babies scream. They are truly gifted at screaming. According to a recent study published in Current Biology, there may be a very good reason for this.
Scientists know surprisingly little about the how or why of screaming, says co-author Dr. David Poeppel, professor of psychology and neural science at New York University. As it turns out, what makes the scream unique isn't related to the usual acoustic factors of volume and pitch. Its effectiveness as an attention-grabber relies on a different quality: roughness.
Humans produce sound by way of vocal cords. Air from the lungs causes the vocal cords to vibrate, sending air-pressure fluctuations traveling through the air as sound waves. The waves' frequency, or speed of vibration, corresponds with pitch. Their amplitude, or strength of vibration, corresponds with loudness.
Loudness can fluctuate, known as amplitude modulation. The researchers put the fluctuation rate of normal speech at about 4 to 5 hertz. When the rate increases to between 30 and 150 hertz, the ear can no longer interpret the changes, resulting in an awful sound. That's "roughness." The study found that within the range of human sounds, only screams have it. It also discovered why that might be.
The study involved a lot of screaming. In one experiment, subjects did the screaming. They screamed syllables. They screamed sentences. They spoke and made vowel sounds. Researchers analyzed the sounds for their measurable acoustic qualities and found that only the screams were rough. In another test, they gathered audio clips from movies and websites and built a database of other people screaming, talking and singing, and of artificial sounds — pleasant music, dissonant music, man-made alarms. Among artificial sounds, both dissonant music and alarms fell in the rough range. Among human sounds, again, only screams were classified as rough, and they were always rough. Roughness seems to be a sound quality reserved for screaming.
It also seems to be the key brain-hijacking effect. It's about fear. When subjects rated the fearfulness of screams, normal speech, screams altered to be less rough and normal speech altered to be rougher, they rated the screams and the artificially rough speech the highest. "The rougher a sound, the scarier or more alarming it [was] rated," says Poeppel.
When researchers monitored subjects' brains during playback, they learned why. Functional magnetic resonance imaging (fMRI) showed the amygdala light up when a subject heard a rough sound. The amygdala is the brain's fear center. It regulates, among other things, the fight-or-flight response. As roughness increased, so did the amygdala's response.
If roughness defines the scream, actively triggering the rescue response in those that hear it, then it's apparent why only screams have it: A scream could literally save a life.
With this kind of evolutionary value, it's clear why screaming is what Arnal calls an "innate communication signal in newborns." Babies are the most helpless of the human species. They need the scream the most.
Something to keep in mind the next time you share a plane with a "crying" infant. The ability to scream bloody murder for two hours straight is good for the species.