Human screams signal more than simply fear and are more acoustically diverse than previously thought, say scientists.
Their results revealed six psycho-acoustically distinct types of scream calls, which indicated pain, anger, fear, pleasure, sadness and joy.
The Swiss research team, whose findings were published in the journal PLOS Biology, also found that, remarkably, non-alarming screams are perceived and processed by the brain more efficiently than screams of alarm.
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They explained that in nonhuman primates and other mammalian species, scream-like calls are frequently used as an alarm signal exclusively in negative contexts, social conflicts or the presence of predators or other environmental threats.
Humans are also assumed to use screams to signal danger and to scare predators. But humans scream not only when they are fearful and aggressive, but also when they experience other emotions such as despair and elation.
Previous studies mainly focused on alarming fear screams, but the new study addressed the knowledge gap using four different psychoacoustic, perceptual decision-making, and neuroimaging experiments in humans.
Twelve participants were asked to vocalize positive and negative screams. A different group of people rated the emotional nature of the screams, classified the screams into different categories, and underwent functional magnetic resonance imaging (fMRI) while listening to the screams.
The results revealed the six psycho-acoustically distinct types of screams. Perhaps surprisingly, listeners responded more quickly and accurately, and with higher neural sensitivity, to non-alarm and positive scream calls than to alarming screams.
Less alarming screams elicited more activity across many auditory and frontal brain regions.
The researchers say their findings show that screams are more diverse in their signalling and communicative nature in humans than frequently assumed.
Study leader Professor Sascha Fruhholz, of the University of Zurich, said: “The results of our study are surprising in the sense that researchers usually assume the primate and human cognitive system to be specifically tuned to detect signals of danger and threat in the environment as a mechanism of survival.
“This has long been supposed to be the primary purpose of communicative signalling in screams.
“While this seems true for scream communication in primates and other animal species, scream communication seemed to have largely diversified in humans, and this represents is a major evolutionary step.
“Humans share with other species the potential to signal danger when screaming, but it seems like only humans scream to signal also positive emotions like extreme joy and pleasure.”
He added: “Signalling and perceiving these positive emotions in screams seemed to have gained priority in humans over alarm signalling.
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“This change in priority might be likely due to the requirements of evolved and complex social contexts in humans.”