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Babies perk up to sounds of ancient hazards

A snake's hiss and angry voices quickly capture infants' attention

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Babies have an ear for primeval dangers, a new study suggests.

By age 9 months, infants pay special attention to sounds that have signaled threats to children’s safety and survival throughout human evolution, say psychologist Nicole Erlich of the University of Queensland, Australia, and her colleagues. Those sounds include a snake hissing, adults’ angry voices, a crackling fire, thunder claps and — as a possible indicator of a nearby but unseen danger — another infant’s cries.

Noises denoting modern dangers, as well as pleasant sounds, failed to attract the same level of interest from 9-month-olds, Erlich and her colleagues report Aug. 27 in Developmental Science.

People can learn to fear just about anything. But tens of thousands of years of evolution have primed infants’ brains to home in on longstanding perils, the scientists propose.

“There is something special about evolutionarily threatening sounds that infants respond to,” Erlich says.

Another study that supported that idea, by psychologist David Rakison of Carnegie Mellon University in Pittsburgh, found that 11-month-olds rapidly learn to associate fearful faces with images of snakes and spiders (SN: 9/26/09, p. 11). “There is now a coherent argument that infants are biologically prepared in at least two sensory systems to learn quickly which evolutionarily relevant objects to fear,” Rakison says.

Further work needs to check an alternate possibility, he adds: that babies learn soon after birth to associate sounds of snakes and other ancient threats with adults’ negative reactions.

Erlich suspects that, at or before birth, babies are already sensitive to sounds that have denoted threats dating back to the Stone Age. The auditory system, unlike the visual system, is nearly mature at birth.

In the new study, 61 male and female infants sitting in highchairs near a parent briefly heard through speakers a series of longstanding danger sounds, modern danger sounds and pleasant sounds. Modern danger sounds included glass breaking and a siren wailing. Pleasant sounds included classical music and a baby laughing.

In response to ancient danger sounds, infants experienced a bigger drop in heart rate, larger eye blinks as measured by electrodes and increased turning toward the speakers or parent, all indicating that they were paying more attention. Babies didn’t cry or otherwise get upset while listening to danger sounds.

Previous studies have found that infants first recognize adults’ expressions of fear between 5 and 7 months of age. Learning what to fear begins around that time, and proceeds more easily and quickly for threats with ancient pedigrees, Erlich proposes.

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