Many hearing parents, hearing professionals, and others forbid the use of sign language with deaf babies and children in favor of speech, deeply ingrained in phonocentric beliefs. As a result, many deaf children are deprived of language.
Linguistics and neuroscience studies show the same timeline and stages of language acquisition in both signing and speech from birth to age 5 and beyond.
Because, language is amodal. That is, language is brain-based, regardless of the modalities (speech or sigining).
At this earliest stage, babies produce coos and goos -- both vocally and manually. They also use communicative gestures including cries for different needs (e.g. when hungry or sleepy).
In the video above, the baby "Juli" manually coos. She was born to ASL-speaking Deaf family members and relatives.
Canonical babbling begins to emerge at about late 6 months or later when babies produce reduplicated syllables in either signing or speech.
In speech, babies produce syllabic consonant-vowel repetitions (vocal babbling). E.g. "dadadada", "babababa" and other meaningless sounds. On the other hand, babies, who are exposed to signing, also produces syllabic hold-movement repetitions (manual babbling).
Neuroscientist Dr. Petitto conducted a scientific study on manual babbling which presented evidence for the natural language in visual-spatial modality.
The video shows the baby "Juli" manually babbling based on the ASL word WORK. "Wait a minute, WORK?" Yes, a long explanation (in another post).
Interestingly, observe the baby babbling with one dominant hand while the other hand is stationary. In WORK, the dominant hand moves while the passive hand is stationary. It's in conjunction with the linguist Robbin Battison's Dominance Conditions.
Finger-pointing (non-linguistic) is a manual gesture emerging at around 9 months regardless of deaf or hearing babies and regardless of manually-speaking or vocally-speaking babies. This is an important evidence for the pronoun acquisition in sign language that manual gesture and signed word are processed differently in the brain.
Raising hand: "Um, why did you use manual gesture because gesture is manual?" <Deconstruction> Because 'gesture' is not tied with hands. There is vocal gesture. Gesture can be either vocal or manual. </Deconstruction>.
At this "sophisticated babbling" stage, babies of English-speaking parents babble constonant-vowel-constonant (CVC). Likewise, babies of ASL-speaking families babble with more varied patterns.
No, babies don't acquire ASL words earlier than English words, contary to the prevalent belief (and myth). ASL babbles are often misinterpreted as first words or earlier words. They are no earlier than spoken babbles "mama".
And, no, it's not easier to manually speak than vocally speak (as explained in another post). The belief of signing being easier is ingrained in phonocentrism.
In general speaking, on the average, babies may utter a few first words up to 10+ words toward their one-word stage at 12 months.
Around the first birthday, babies produce new referential one-word utterances and gradually expand more and more words on a daily basis. Same in both signing and speech.
Babies don't produce words like adults do but they can convey them consistently to express meanings.
For example in English speech, a baby says "mu" every time she/he is offered a bottle of milk. Or, a baby utters "open handshape tapping on torso" meaning a "bath" when she/he is prepared for a bath.
Like English-speaking people, ASL-speaking adults utter regular ASL words with babies. They don't adapt nor simplify these ASL words. Again like speech, parentese (or motherese) do exist in signing.
At this two-word stage, toddlers are able to utter words more identifiable. They can express over 50-200 words and make two-word combinations (e.g. "Where ball?", "More pull", "daddy car", "where dog", "water on").
Sometimes, toddlers don't speak (either manually or vocally) until age 2 or so and then suddenly boom with a bunch of words. Or, because babies are deaf with no exposure of signed language. Toddlers may speak fewer than 50 words in either modalities/languages.
An important evidence for natural language is the pronoun acquisition which emerge at about 18 months (usually with reversal error) towards the proper use at about 24 months. E.g. When a mother says "Do you want a drink?" to the toddler, the toddler thinks they can use "you" referring to themselves, "Yes, you want drink" [reversal error]. Then the mother says, "No, I don't want a drink. You want a drink?" The toddler eventually figures out the concept of a pronoun.
Dr. Petitto asked a question if ASL-speaking children acquire ASL pronouns earlier than English pronouns? Because, the ASL pronouns are iconic. Index finger pointing toward oneself (I/me) or toward the opposite person (you).
In her study, the answer is (surprisingly or not surprisingly) no. In my study of my own child's ASL language acquisition, I can also confirm that it's true. And, pronoun acquisition in ASL and English are on the same timeline. This illustrates that:
1) ASL or signing is not easier than English or speech; 2) ASL is not gestural but a natural human language, 3) language is amodal (brain-based language, independent from modality), 4) manual gesture and signed words are processed differently in the brain.
Speaking of the #4 above, remember the pointing gesture at 9 months? The movement and handshape of the ASL pronouns (which emerge at about 18 months) are the same as the gestural finger-pointing (which emerges at about 9 months). Yet, they function differently, despite the same direction and handshape.
At the stages of age 2 and beyond, the concepts are basically the same for grammar, prepositions (e.g. in, out, under, etc.), three-word stage, complex-utterance stage, the concept of past/future time, and so on beyond age two in both modalities (visual-spatial and vocal-aural) as described in other posts.
Based on the first two years of milestones, they show enough evidence that 1) language is amodal that language is not central to speech, 2) signing and speech are on the same timeline of language acquisition (that is, amodal), 3) signed languages and spoken languages are equal.