Fascinating little tidbit that I ran across today: In 1854, 1 in 155 residents of Martha's Vineyard were deaf—compared to 1 in 5728 as the national average.
Historians trace those high rates of deafness back to a genetic variation common in Weald, England. People from this rural, sparsely populated region moved to Martha's Vineyard in the late 1600s, where they joined a pretty genetically isolated population, with few off-island marriages. The result was a high rate of this specific kind of deafness. That's interesting enough, but what's really amazing is how the genes shaped culture.
Until the 20th century, deafness was an unremarkable, normal part of life on the island—akin to the level of "oddness" or handicap we'd ascribe to left-handedness today. Pretty much everyone, deaf or hearing, spoke a local version of sign language, which made it possible for the deaf to be fully integrated into society without anybody really missing a beat.
Martha's Vineyard Sign Language is mostly dead today, but it has an important legacy. In the early 19th century, children from the island brought their language to America's first school for the deaf, where it mingled with French Sign Language and other colloquial home sign traditions to create modern American Sign Language.