Babies exposed to sign language babble silently with their hands even if they can hear, US research shows.
Babies babble with their hands as well as their mouths
Dr Laura-Ann Petitto at Dartmouth College, Hanover, had previously found similar hand-babbling in deaf babies.
But critics say deaf babies cannot be compared directly with hearing babies and babbling should not be regarded as an attempt at language.
In New Scientist, Dr Petitto says her research proves babbling - vocal or hand - is an active language attempt.
Most babies make a babbling 'ba, ba, ba' sound at around seven months.
Some scientists say this is merely a motor activity driven largely by the baby's emerging control over the movement of their mouth and jaw.
Others believe it is an attempt to mimic human speech and reflects the baby's innate sensitivity to the rhythm of language.
Dr Petitto has argued that deaf babies who are exposed to sign language learn to babble using their hands in the same way that hearing babies learn to vocally babble with their mouths.
Her latest research, soon to be published in the journal Cognition, shows hearing babies exposed to sign language also begin to babble with their hands.
Dr Petitto and colleagues studied six babies with normal hearing at six, 10 and then 12 months of age.
Half of the infants were only exposed to sign language because both of the parents were deaf and did not use speech.
The other three infants had hearing, speaking parents.
The researchers theorised that if babbling was merely a motor activity, as their critics suggested, the babies exposed to sign language alone would display similar hand movements to the babies exposed to speech.
When they analysed the babies' hand movements they found big differences between the two groups.
The babies exposed to sign language made hand movements that resembled those made by their deaf parents.
Dr Petitto said this confirmed that babbling was a linguistic activity.
The findings would not be possible unless all babies were born with a sensitivity to specific rhythmic patterns at the heart of human language and the capacity to use them, she said.
Professor Jim Kyle, from the Centre for Deaf Studies at the University of Bristol, has conducted similar research in the UK.
He said Dr Petitto's findings were plausible.
"It's absolutely right that deaf babies of deaf parents babble - using rhythmic movements that seem to be in tune with the parents' movements.
"It does seem right that deaf babies move their hands rhythmically in the same way that hearing children would do [with their mouths] in speech babble. That's pretty reasonable," he said.
But he said it was more complicated to study hearing babies of deaf parents.
"It's problematic to get a situation where a deaf mother only uses sign with a hearing baby. Deaf parents will often try to talk to their baby rather than sign so we have never really got pure data.
"But it's plausible that a hearing baby could babble like a deaf baby given the right sort of circumstances and that would logically be an argument that all languages babble," he said.
A spokeswoman from the RNID said: "We welcome the research, especially because British Sign Language was only recognised as an official language in March last year.
"This kind of research is great. It's good that it draws parallels with oral language," she said.