All babies use their hands to make gestures, the researchers note. But the deaf children also repeatedly practiced and experimented with the manual signs they obviously saw their parents using in ways that the hearing babies did not. And just as the hearing children strung their babbling syllables into longer sequences of sounds that had the same rhythm and length as spoken sentences, so the deaf babies' manual babbling progressed to resemble ASL sentences.

"Like other languages, sign language is governed by linguistic rules," said Dr. Richard Kretschmer, professor of early childhood development at the University of Cincinnati. "Deaf children are not born with some innate ability to learn it. All children experiment with language before they use it, so it's logical that a child exposed to sign language will experiment too."

Manual babbling, however, challenges traditional notions that language skills are dependent on the development of speech and the maturation of the neural mechanisms that control the vocal cords.

We further analyzed all manual activities that were not ASL signs and were not pointing to objects to determine whether they had any systematic organization. If so, we analyzed these activities to determine whether they had unique organizational properties or whether they shared phonetic and syllabic organization common to signed languages. Attribution of manual babbling was applied only to forms that fulfilled the same criteria as vocal babbling. This transcription system permitted direct comparisons of the manual activities of the deaf and hearing infants.