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Development
Joseph Garcia, an American Sign Language (ASL) interpreter and a leading proponent of use of ASL in communicating with infants and toddlers, began with his graduate thesis in 1986, an analysis of the role sign language could play in early childhood language acquisition. His research indicated babies who are exposed to signs regularly and consistently at six to seven months of age can begin using signs effectively by the eighth or ninth month.

Research by Bonvillian & Folven indicates that children raised in a signing environment produce their first signs at a mean age of 8.2 months, whereas Capute et al. found a mean age of 11.3 months for the production of the first word in speaking children. This acquisition advantage has been found to extend to multi-word and multi-sign milestones as well. Signing children acquired 10 sign vocabularies at a mean age of 13.1 months, compared with 10 word vocabularies at a mean age of 15.1 months for speaking children. Milestones of 50 signs and 50 words were acquired at mean ages of 18 months and 19.6 months respectively.

Two-sign combinations were first produced at a mean age of 17.1 months, while two-word combinations were first produced between 18 and 24 months. Acquisition of morphology or inflection was not found to differ greatly between signing and speaking children.

In 1998, a program was conducted at A. Sophie Rogers Infant-Toddler Laboratory School in Ohio State University by Kimberlee Whaley. Infants as young as 9 months old and their teachers began to learn to use some signs from the American Sign Language to communicate with each other. The program was not intended to teach American Sign Language, rather to use signs to communicate effectively. The program found that children would use the signs they learned in the classroom at home. Another finding indicated that girls use signs more than boys.

Controversy
Researchers have suggested the possibility of parents and experimenters being overly liberal in attributing sign or word status to early attempts at communication by children. There is not a universal consensus on the criteria for differentiating a child's sign or word from a more simple gesture or vocalization. Large differences in acquisition arise when only signs that label objects rather than request objects are considered. In this case the mean age of first sign production becomes 12.6 months and is more comparable with the mean age of first word production in speaking children (11.3 months).

Reasons for Differences
Two possible explanations have been suggested for the differences shown in language acquisition between speaking and signing children.

Multiple Timing Mechanism

Under this theory, separate mechanisms would control vocabulary and syntax development. Earlier maturation of the visual system as compared with the oral system would result in fewer constraints on the production of a first sign vs. production of a first word and also allow for the separate acquisition of vocabulary and syntax.

Unitary Timing Mechanism

This mechanism would be activated at the onset of language learning and would result in a relatively fixed amount of time between language acquisition milestones. Alternatively, a unitary mechanism could activate at the same time in all children, but environmental or peripheral factors, such as language modality or differences in a language's morphology and syntax could explain the differences between acquisition milestones in speaking and signing children.