Mayberry, I. R. (Dec 1993). "First-language acquisition after childhood differs from second-language acquisition: the case of American Sign Language". Journal of Speech and Hearing Research. 36 (6): 1258–70. doi:10.1044/jshr.3606.1258. PMID8114493.
"Sign Language Stated in Law!" at jfd.or.jp; excerpt, "Every person with disabilities, wherever possible, shall be ensured opportunities to choose his or her language (including sign language) and/or other means of communication, and the expansion of opportunities to choose his or her means of acquiring or utilizing information shall be promoted."
Mayberry, I. R. (Dec 1993). "First-language acquisition after childhood differs from second-language acquisition: the case of American Sign Language". Journal of Speech and Hearing Research. 36 (6): 1258–70. doi:10.1044/jshr.3606.1258. PMID8114493.