Emory University psychologist and scream researcher Harold Gouzoules says one thing's for sure: The ability to both scream and respond to a scream is deeply rooted in our evolutionary history.
"Screams happen, evolutionarily, when you're in trouble," he said. "In primates, screaming primarily occurs in the context of fights."
But in humans, he says, the reason for screaming has been broadened.
"We too scream in an aggressive context, when we're angry," he said. "But we also scream in fear, pain, surprise and when we're startled or excited. Are these acoustically different, and can we tell?"
Gouzoules gathers screams from all over, including movies, TV shows and YouTube videos, and puts them on audio files. Then he presents them to volunteers in his lab to see whether they can tell the type of scream without any visual clues. He finds that people can easily distinguish between a scream, a moan and a yell, and most can tell the difference between screams of fear and aggression. The most difficult to distinguish are screams of fear and excitement.
"My hypothesis is that people with the most empathy for others are best at deciphering the emotions behind screams," Gouzoules said. "And the literature suggests that women are better than men. I suspect that's because women are better at processing emotional cues."
But just how do screams scare us?
suggests that screams target a special acoustic "sweet spot" in our brain, designed to snap us to attention to possible danger.
"If you ask a person on the street what's special about screams, they'll say that they're loud or have a higher pitch," said David Poeppel, professor of psychology and neural science at New York University. "But there's lots of stuff that's loud and there's lots of stuff that's high-pitched, so you'd want a scream to be genuinely useful in a communicative context."
By plotting sound waves of various screams in a similar manner to auditory neurons, Poeppel and colleague Luc Arnal discovered that screams occupy a reserved chunk of the auditory spectrum not used by normal speech, which they dubbed a "privileged niche" in human communications.
"We even saw that this observation remained true when we compared screaming to singing and speaking, even across different languages," said Arnal, a neuroscientist at the University of Geneva.
What contributes to that niche, the authors said, is that screams have a unique vocal quality: roughness. But what's "rough" about a scream? For most of us, roughness might indicate a raspy, harsh or gravelly sound or voice. However, in psychoacoustics, roughness equals dissonance, or the unpleasant qualities of a sound.
"Roughness refers to fast sound changes in loudness (in a 30 to 150 hertz frequency range) of a sound that can be high-pitched or not," Arnal said. If that isn't clear, he added, then think of roughness like a strobe light.
"Everyone is familiar with those lights that flash superfast in clubs," Arnal said. "Screams could be considered as strobophones, since they are modulating very fast in an analogous way in the auditory domain."
Study subjects were asked to listen to different human screams as well as artificial alarms, like buzzers, car alarms and horns, and compare them with control sounds. Researchers found that the screams and sounds rated "roughest" were the ones that were most terrifying to the listener.
"We found that the rougher the sound, the stronger the response in the amygdala, the part of the brain involved in fear reactions," Arnal said. The researchers also found they could make a nonscream more terrifying and scream-like by adding roughness.
What was also "peculiar and cool" about their findings, the researchers said, is that alerting signals like car alarms and house alarms activated the same auditory range as screams.
"These findings suggest that the design of alarm signals can be further improved," Poeppel said. "The same way a bad smell is added to natural gas to make it easily detectable, adding roughness to alarm sounds may improve and accelerate their processing."
Wonder what the next generation of Hollywood scream queens might do with that information.
"Well, yes, our results definitely show that rough screamers are the best candidates to make great horror movie actresses and actors," Arnal said.