I've been fascinated by the history and development of sign language for a while now. Highly linked to local Deaf cultures, individual sign languages have deep roots in the home-made systems people came up with in order to communicate with one another and with their families at times when Deaf people were often a lot more socially isolated than they are today. That means that each sign language is unique — even British and American sign language aren't at all the same thing. English is spoken in both countries, but the cultural history that gave birth to sign was sufficiently different to produce two completely different languages that are unintelligible to one another. (Meanwhile, American sign language is much closer to French, because it also has roots in a system imported from France in the 19th century.)
In that case, it was a physical distance that lead to the development of two different sign languages. But, within the United States, the same thing happened because of social distance. Turns out, there is a Black American sign language that is distinctly different, as a language, from ASL. Its roots lie in segregation, and especially in separate-and-not-at-all-equal school systems. Ironically, though, that meant sign language had a more prominent place in black schools for much of the 20th century. At white schools, up until the 1970s and 1980s, students were heavily pressured to speak and lip-read, rather than sign — because it was thought to be better. Meanwhile, at black schools, sign language continued to be heavily used, growing and changing. By the late 1960s, the two systems were almost completely different languages.
Carolyn McCaskill remembers exactly when she discovered that she couldn't understand white people. It was 1968, she was 15 years old, and she and nine other deaf black students had just enrolled in an integrated school for the deaf in Talledega, Ala.
… The teacher's quicksilver hand movements looked little like the sign language McCaskill had grown up using at home with her two deaf siblings and had practiced at the Alabama School for the Negro Deaf and Blind, just a few miles away. It wasn't a simple matter of people at the new school using unfamiliar vocabularly; they made hand movements for everyday words that looked foreign to McCaskill and her fellow black students.
…So, McCaskill says, "I put my signs aside." She learned entirely new signs for such common nouns as "shoe" and "school." She began to communicate words such as "why" and "don't know" with one hand instead of two as she and her black friends had always done. She copied the white students who lowered their hands to make the signs for "what for" and "know" closer to their chins than to their foreheads. And she imitated the way white students mouthed words at the same time as they made manual signs for them.
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Via Stan Carey