Do deaf babies babble with their hands, and what if nobody signs back?

Yes. Babies who grow up with a sign language produce manual babbling, the hand version of the babbling of hearing babies. They then make their first sign and their first two-sign sentences on the same timeline as hearing children acquiring a spoken language. Language, it turns out, is not tied to the mouth: a baby learns whichever language it can perceive.

The difficulty is access. Deaf children born to deaf parents are usually exposed to a sign language at birth. But at least 90% of deaf children are born to hearing parents who use a spoken language at home. Hearing loss prevents many of them from hearing spoken language well enough to acquire it, so for many, language acquisition is delayed until they meet a sign language or begin using hearing aids or cochlear implants.

Why that matters comes down to timing. Research suggests the critical period for language applies to sign language too. Studies found that deaf adults first exposed to sign after age 5 had notably weaker grammar in it than those exposed from birth, and weaker still when exposure came after age 8. Researchers use the term language deprivation for a lack of the language input a child needs to acquire a language.

What families should do is debated. Some researchers argue early sign language does not hinder later spoken language. Other clinical research on children with cochlear implants found that those without exposure to sign were almost twice as likely to reach age-appropriate spoken language. In either case, the research finds that exposure to fluent language models lowered the risk of language deprivation.