Melbourne, Aug 2 (PTI): Mothers change their voice when talking to their babies to provide comfort and appear less threatening to offspring, according to a study which suggests that 'baby talk' is an important part of human speech evolution.

Through the subconscious mechanism of babytalk, a parent's voice can offer encouragement, discipline or comfort, and even facilitate early language development in infants, researchers said.

The study by researchers at Western Sydney University in Australia shows that mothers unconsciously shorten their vocal tract when speaking to their babies, creating a higher pitch that is thought to have evolved from pre-speech that primate ancestors used to appear less threatening to offspring.

It was only once human language emerged that babytalk, formally referred to as Infant Directed Speech (IDS), acquired a second purpose - to facilitate language learning in infants, said Marina Kalashnikova, who led the research.

"Infant Directed Speech is actually a powerful tool that parents instinctively use to aid language development in their infant's first months and years of life," said Kalashnikova.

"Shortening of the vocal tract is not unique to humans – it is an adjustment that several species make to appear smaller and less threatening," she said.

"But specifically for humans, by shortening their vocal tract, mothers produce clearer speech sounds - especially vowel sounds," she added.

"Mothers' speech also sounds more similar to an infants' own vocalisation and this has been proposed to drive their preference for IDS; further, infants prefer to listen to speech that is similar to the sounds that they produce," Kalashnikova said.

In comparison to adult-directed speech, IDS has simpler grammar, more varied pitch, longer pauses, greater emotion and inflection, as well as distinguishable speech sounds and exaggerated facial expressions.